What is disease? Malfunctions, pathogens and the three kinds of invader
By the end of this lesson you will be able to
- Define disease, infectious (communicable) disease, micro-organism and pathogen, and distinguish an illness caused by a bodily malfunction from one caused by a pathogen.
- Describe fungi, bacteria and viruses as the three main types of pathogen, and compare their size, structure and way of reproducing.
- Explain how each type of pathogen makes a person ill, at the level of cells.
- Outline why bacteria and viruses change so quickly, through rapid multiplication and mutation.
- Identify the pathogen type of six named diseases — athlete’s foot, ringworm, salmonella, MRSA, HIV and avian flu H5N1 — and apply that knowledge to an unfamiliar case.
The puzzle
Four people are ill in one house in Serowe. A grandmother’s blood sugar is too high and she needs insulin every day. A boy has a red, itchy ring on his arm that appeared a week after he started helping with the calves. His father has been vomiting since a lunch of chicken that was still pink at the bone. And the baby has a fever, a cough and a runny nose that half the crèche seems to share. All four are “sick”. But only three of these illnesses could be caught by a visitor — and one of them cannot be cured by any medicine on Earth, yet will be gone in a week. What is going on in each body, and why does it matter which is which?
Sort the four illnesses into two groups: “something in the body has stopped working properly” and “something has got into the body and is multiplying”. Then predict which of the second group could be treated with an antibiotic — and which could not.
Compare your answer
The grandmother’s diabetes is a malfunction: her own pancreas no longer makes enough insulin, and nothing has invaded her. The other three are invasions — a fungus growing in the boy’s skin, salmonella bacteria multiplying in the father’s gut, a cold virus copying itself inside the cells of the baby’s nose. Only the father’s bacterial infection could be touched by an antibiotic (and even then doctors usually let the body clear salmonella itself). The fungus needs an antifungal cream; the virus needs nothing but time, because antibiotics do nothing whatever to viruses. Knowing which invader you are dealing with is the first decision in medicine, and it is the subject of this unit.
The science — structured notes
1.1 Health, disease and two ways a body goes wrong
Health is more than not being ill: the World Health Organization defines it as complete physical, mental and social well-being. A disease is any condition that stops part of the body working as it should, with signs (what a doctor can measure — a temperature, a rash) and symptoms (what you feel — pain, tiredness). Diseases come in two families, and the difference decides everything about how they are treated.
| Non-infectious disease — a bodily malfunction | Infectious (communicable) disease — caused by a pathogen | |
|---|---|---|
| What has gone wrong | The body’s own cells, organs or chemistry are not working properly | A living thing (or a virus) has got into the body and is multiplying there |
| Can it be passed on? | No — you cannot catch it from another person | Yes — it can be transmitted from one person (or animal) to another |
| Causes | Genes (sickle-cell anaemia), diet (scurvy, from too little vitamin C), lifestyle and age (type 2 diabetes, heart disease), the immune system attacking the body (type 1 diabetes, asthma), uncontrolled cell division (cancer), injury | Fungi, bacteria, viruses (and protists such as the malaria parasite) |
| Examples | Diabetes, asthma, heart disease, cancer, scurvy, a broken leg | Athlete’s foot, ringworm, salmonella, MRSA, HIV, flu, TB, cholera, measles |
| Treatment aims to | Replace what is missing (insulin), repair, remove or manage | Kill or remove the pathogen, or help the body do so, and stop it spreading |
1.2 Micro-organisms: mostly harmless, sometimes essential, occasionally deadly
A micro-organism (microbe) is any living thing too small to see without a microscope: bacteria, single-celled fungi such as yeast, microscopic algae and protists, and — stretching the word, since they are not alive in the usual sense — viruses. Your body is home to about 38 million million bacteria, mostly in your gut, slightly more than the number of your own cells. They digest fibre you cannot, make vitamin K, train your immune system and crowd out invaders. Yeast raises bread and brews; bacteria turn milk into madila and yoghurt; fungi and bacteria in soil decompose every dead leaf and cow pat on the planet and return the nutrients. Without micro-organisms there would be no soil and no life on land.
A pathogen is the exception: a micro-organism that causes disease in the organism it lives in (its host). Fewer than one species in a thousand is a pathogen of humans, but that small group has killed more people than every war in history.
“All bacteria are pathogens, so the best thing for your health would be a body with no bacteria at all.”
Reveal answer
False, twice. Most bacteria are harmless, and the ones in your gut are essential — they digest fibre, make vitamins and occupy the space a pathogen would need. People who take a long course of strong antibiotics often get diarrhoea or thrush precisely because the helpful bacteria have been killed and something else has moved in. A body with no microbes is not healthier; it is defenceless.
1.3 The three kinds of invader, compared
The three pathogens in this unit differ in size by a factor of a thousand, and in structure completely. Get the sizes into your head with the units: 1 mm = 1000 μm (micrometres) = 1 000 000 nm (nanometres). A human cheek cell is about 60 μm across; a full stop on this page is about 500 μm.
| Fungi | Bacteria | Viruses | |
|---|---|---|---|
| Size | Yeast cells 3–10 μm; thread-like hyphae 5–10 μm wide but millimetres long | 0.5–5 μm (an E. coli cell is about 2 μm long) — up to a thousand fit across a full stop | 20–300 nm — a flu virus is about 100 nm, so about 20 fit across one bacterium |
| Is it a cell? | Yes — eukaryotic cells with a nucleus, like ours, but with a cell wall of chitin | Yes — a prokaryotic cell: no nucleus, a loop of DNA, a cell wall, sometimes a flagellum (Year 1 Unit 5) | No — a strand of genetic material (DNA or RNA) inside a protein coat, sometimes with a fatty envelope; no cytoplasm, no membrane of its own, no metabolism |
| Is it alive? | Yes | Yes | Not on its own: it cannot feed, grow, respire or reproduce outside a host cell |
| How it reproduces | Spores (blown, or shed from skin) that grow into new hyphae; yeasts bud | Binary fission: one cell copies its DNA and splits into two — every 20 minutes in ideal conditions | Enters a host cell and forces the cell’s own machinery to make thousands of copies, which burst out or bud off |
| Seen with | Light microscope (×100–400) | Light microscope at ×1000, just; colonies with the naked eye | Electron microscope only |
| Diseases in this unit | Athlete’s foot, ringworm | Salmonella (food poisoning), MRSA | HIV, avian influenza H5N1 |
| Killed by | Antifungal medicines | Antibiotics (unless resistant); disinfectants; heat | Antivirals slow them; the immune system and vaccines are the main defence; soap and disinfectants destroy them outside the body |
How many flu viruses (100 nm) would fit end to end across an E. coli cell (2 μm)? How many E. coli across a full stop (0.5 mm)?
Convert to the same unit first. 2 μm = 2 × 1000 nm = 2000 nm.
Viruses across a bacterium: 2000 nm100 nm = 20.
Full stop: 0.5 mm = 500 μm. Bacteria across it: 500 μm2 μm = 250. So about 250 × 20 = 5000 viruses would fit across the full stop — which is why a sneeze can carry millions of them in a single droplet.
A laboratory receives three samples. Sample 1 shows rod-shaped cells 2 μm long, with no nucleus, dividing in two under the microscope. Sample 2 shows nothing at ×1000 but, under an electron microscope, 90 nm particles with a spiky coat. Sample 3 shows branching threads 8 μm wide with nuclei visible after staining. Identify each pathogen type and give the observation that decided it.
Reveal answer
Sample 1: a bacterium — a cell with no nucleus (prokaryote), micrometre-sized, reproducing by binary fission. Sample 2: a virus — far too small for a light microscope (nanometres), a particle with a protein coat rather than a cell. Sample 3: a fungus — thread-like hyphae made of eukaryotic cells with nuclei, micrometres wide. The decisive observations are size (nm versus μm) and whether there is a nucleus.
1.4 How each pathogen makes you ill
1.5 Why pathogens change so fast: multiplication and mutation
A bacterium that divides every 20 minutes produces, from one cell, 8 in an hour, 512 in three hours and over a million in seven — Lesson 2.2 does the arithmetic. Each division copies the DNA, and copying is not perfect: roughly one mistake in every thousand million letters copied. That sounds tiny, but a million million bacteria in an infected gut are making a million million copies a day, so every possible single mistake happens somewhere every day. A mistake that is copied into the next generation is a mutation. Most do nothing or kill the cell; a rare one makes the cell survive an antibiotic, or dodge the immune system, and that cell’s descendants take over. Viruses copy even more sloppily — flu makes a mistake every few thousand letters — which is why the flu vaccine has to be redesigned every year and why H5N1 is watched so closely (Lesson 2.5). Change, the key concept of this unit, is not something pathogens do occasionally; it is what they do fastest.
A doctor says: “Bacteria evolve resistance in weeks; elephants take a million years to change.” Using the ideas of multiplication and mutation, explain why the speed of change is so different.
Reveal answer
Change happens by mutation followed by the survival of whichever variant copes best, and it accumulates generation by generation. A bacterium has a generation every 20 minutes and a population of millions of millions, so millions of mutations are tested every day and a useful one can spread through the population in days. An elephant has a generation every 25 years and populations of thousands, so the same number of “tries” takes hundreds of thousands of years. Pathogens change fast because they multiply fast and in enormous numbers, not because their mutations are special.
1.6 The six diseases you must know
| Disease | Pathogen (type) | How it spreads | What it does | Treatment and prevention |
|---|---|---|---|---|
| Athlete’s foot | Trichophyton species (fungus) | Contact: wet floors at pools and showers, shared towels and shoes | Itchy, cracked, peeling skin between the toes | Antifungal cream or powder; dry feet, own towel, sandals in communal showers |
| Ringworm | Trichophyton and Microsporum species (fungus) | Skin contact with an infected person, calf, dog or cat; towels, combs, floors | A red, scaly ring that spreads outwards; on the scalp, patches of hair loss | Antifungal cream or tablets; wash hands after handling animals; do not share combs |
| Salmonella food poisoning | Salmonella enterica (bacterium) | Undercooked chicken and eggs, unpasteurised milk, food handled with unwashed hands after the toilet | Diarrhoea, vomiting, cramps and fever 6–72 hours after eating; dangerous mainly through dehydration | Fluids and salts (oral rehydration); antibiotics only for severe cases; cook poultry to 75 °C throughout, keep raw meat separate, wash hands |
| MRSA | Methicillin-resistant Staphylococcus aureus (bacterium) | Contact: skin, wounds, hospital surfaces and hands; often carried harmlessly in the nose | Wound and skin infections, abscesses; in the blood or lungs it can kill because the usual antibiotics no longer work | A few reserve antibiotics; hand hygiene in hospitals, screening patients, wound care |
| HIV infection | Human immunodeficiency virus (virus) | Body fluids: sexual contact, shared needles, from mother to baby in pregnancy, birth or breastfeeding; never by touch, air, food or mosquitoes | Infects and destroys helper white blood cells; over years, without treatment, the immune system fails (AIDS) and other infections take hold | Antiretroviral medicines (ARVs) taken daily keep the virus so low that the person stays healthy and cannot pass it on; condoms; testing; treatment in pregnancy |
| Avian influenza H5N1 | Influenza A virus, subtype H5N1 (virus) | From infected birds (and now some mammals) to people who handle them; so far it very rarely passes between people | Severe flu and pneumonia — roughly half of the few hundred people known to have caught it since 2003 have died | Antivirals early; culling infected flocks; the fear is a mutation that lets it spread between people |
A poster in a clinic reads: “HIV is a bacterium spread by mosquitoes and by sharing cups. Antibiotics cure it if taken early.” Identify three errors and correct each.
Reveal answer
HIV is a virus, not a bacterium — a particle about 120 nm across with no cell structure. It is spread by body fluids (sexual contact, blood, mother to child), not by mosquitoes, cups, touch or air — a claim that spreads fear and stigma. Antibiotics do nothing to any virus; HIV is controlled (not cured) by antiretroviral medicines, which keep the virus so low that the person lives a normal life and cannot pass it on.
A new disease appears and doctors do not yet know what causes it. List three things they would look for under the microscope, or test for, to decide whether the pathogen is a fungus, a bacterium or a virus — and say why the answer changes the treatment.
Reveal answer
Size: something visible at ×400–1000 is a cell (fungus or bacterium); something invisible until the electron microscope is a virus. A nucleus after staining means a fungus (eukaryote); no nucleus means a bacterium (prokaryote). Whether it grows on nutrient agar on its own: bacteria and fungi do, viruses cannot grow without living cells. Whether antibiotics stop it in a dish: only bacteria. The treatment depends on the answer because antibiotics work only on bacteria, antifungals only on fungi, and antivirals only on the specific virus they were designed for.
Try it — one cell becomes a colony
Spread a drop of diluted yoghurt on nutrient agar and incubate it. Every colony you count grew from one bacterium that was alive and able to divide — so the count is a count of cells, made visible by their own multiplication. Change the temperature from 5 °C to 65 °C and watch what happens to the count; then switch the agar to plain and see what a cell without food can do.
Suppose your results after 24 h on nutrient agar were: 5 °C → 6 colonies, 15 → 38, 25 → 91, 35 → 118, 45 → 108, 55 → 24, 65 → 0.
(a) Describe the relationship, quoting values.
(b) Explain, at the level of the cell, why the count rises to about 35–40 °C and then collapses.
(c) Explain why school laboratories incubate plates at 25 °C and never at 37 °C, and why plates are taped shut.
Reveal answer
(a) The colony count rises with temperature from 6 at 5 °C to a peak of 118 at 35 °C, then falls steeply to 24 at 55 °C and 0 at 65 °C — a hill shape, not a straight line.
(b) A bacterium grows and divides using enzyme-controlled reactions, which speed up as the cell warms, so more cells manage to divide enough times in 24 h to form a visible colony. Above about 45 °C the enzymes are denatured — their shape is destroyed by heat — the cell can no longer make what it needs, and it dies; at 65 °C nothing survives.
(c) 37 °C is human body temperature: a plate incubated there favours exactly the bacteria that grow best in people, including any pathogen that landed on it, and lets them multiply to dangerous numbers. At 25 °C harmless environmental bacteria grow well enough to count while body pathogens grow poorly. Taping the lid stops the plate being opened, so that spores and cells cannot escape into the air or onto hands — but the tape is not sealed all round, because a fully sealed plate can favour bacteria that grow without oxygen, some of which are dangerous.
Common mistakes to avoid
- “Germs” or “bugs” — say fungus, bacterium or virus, and “pathogen” for the group.
- “Viruses are tiny bacteria” — a virus is not a cell at all; a bacterium is a complete living cell.
- Mixing up μm and nm — 1 μm = 1000 nm; bacteria are micrometres, viruses nanometres.
- “Antibiotics cure infections” — bacterial infections only; they do nothing to a virus or a fungus.
- “Ringworm is a worm” — it is a fungus growing in the skin.
- “All diseases can be caught” — diabetes, asthma, cancer and scurvy are malfunctions; nobody catches them.
- “Every microbe is harmful” — most are harmless or essential; a pathogen is the rare exception.
How we found out — seeds of disease, animalcules and Koch’s rules
For most of history disease was blamed on bad air (miasma), on the stars, or on sin. In 1546 the Italian physician Girolamo Fracastoro proposed something new: that contagious diseases were spread by tiny “seeds” (seminaria) that could pass from person to person by touch, on clothing, or through the air. He could not see them, and nobody believed him for three hundred years. In 1676 the Dutch cloth merchant Antonie van Leeuwenhoek, grinding his own lenses, saw “animalcules” in a drop of pond water, and in 1683 drew the first bacteria from the plaque on his own teeth — but he never connected them to disease, and neither did anyone else for nearly two centuries.
The connection was made by the German doctor Robert Koch. In 1876 he grew the anthrax bacterium outside an animal, injected it into mice, and recovered the same bacterium from the dead mice — proof that one specific microbe caused one specific disease. On 24 March 1882 he announced the bacterium that causes tuberculosis, then the leading cause of death in Europe, and in 1884 the one that causes cholera. Out of this work came Koch’s postulates, the rules for proving a microbe is a pathogen: it must be found in every case of the disease and not in healthy hosts; it must be grown in pure culture; the culture must cause the disease when introduced into a healthy host; and it must be recovered again from that host. Koch’s assistant Julius Petri designed the dish, and Koch’s laboratory used agar — suggested by Fanny Hesse, the wife of another assistant, who used it to set jellies — as the growth medium. The Petri dish of nutrient agar in the lab above is their invention, unchanged in 140 years.
Fracastoro’s “seeds” and Koch’s postulates both say that microbes cause disease. Explain why Koch’s work changed medicine and Fracastoro’s did not.
Reveal answer
Fracastoro made a claim with no evidence anybody could check: the seeds were invisible and he had no way to show they existed or that they caused anything. Koch supplied a method: isolate the microbe, grow it alone, give it to a healthy animal, watch the disease appear, recover the microbe again. Anyone with a microscope and a dish could repeat it and either confirm or refute it — and the postulates gave every later disease a test to pass. Science changes minds with evidence that can be repeated, not with ideas that later turn out to be right.
Where it is used now
- Botswana’s cattle. Foot-and-mouth disease is a virus of cloven-hoofed animals; one outbreak can close beef exports for a year. The veterinary cordon fences that cross the country, the vaccination of herds in the north and the movement permits for cattle are all pathogen control — an interaction between people, their animals and a virus that decides livelihoods.
- Tuberculosis. Koch’s bacterium is still with us: Botswana has one of the higher TB rates in the world, made worse by HIV, which weakens the defences that normally keep TB dormant. Diagnosing it means finding the bacterium in sputum — the same test Koch used, now done by a rapid molecular machine in under two hours.
- Malaria in the north. Not a fungus, bacterium or virus but a protist, Plasmodium, carried by mosquitoes; Botswana’s Okavango and Chobe districts still report cases every rainy season. Knowing the pathogen type is why the treatment is an antimalarial and the prevention is a bed net.
- Food safety. Every abattoir, dairy and restaurant kitchen is designed around bacteria: cooking temperatures, fridge temperatures, separate boards for raw meat. Salmonella and E. coli are the reasons.
- Hospitals. MRSA is why every ward has alcohol gel at the door and why surgeons scrub for minutes: the bacterium that ordinary antibiotics cannot kill is stopped by soap and by not being carried in.
Science now — and the scientist you need to be
What is happening in 2026
Koch needed weeks to grow and identify a pathogen; a laboratory today can read the complete genetic code of every microbe in a sample — a technique called metagenomic sequencing — in a day, and name a pathogen nobody has seen before. AI systems trained on millions of microscope images now sort bacteria by shape and staining faster than a technician, and flag the unusual ones for a human to look at. In 2025 researchers reported AI models that read a bacterium’s genome and predict which antibiotics it will resist before any culture has grown, so that a patient with sepsis can get the right drug in hours instead of days. And the boundary of what counts as a pathogen keeps moving: the fungus Candida auris, unknown before 2009, is now on the WHO’s list of most dangerous fungal pathogens because it resists most antifungal drugs and spreads in hospitals. Every one of these advances rests on the distinctions in this lesson — cell or not, nucleus or not, μm or nm — because those are what decide what will kill it.
The mindshift
What you need to be able to do
Criterion A questions in this unit start with an unfamiliar disease or scenario — a sick calf, a poster, a laboratory report — and ask you to classify the pathogen and explain the consequences. The examiners’ reports say the same thing every session: give the mechanism, not the label. “It is a virus” scores one mark; “it is a virus, so it has no cell wall or ribosomes of its own for an antibiotic to attack, and it multiplies only inside the host’s cells” scores full marks.
See an example
Cupboard: paracetamol (malfunction — pain and fever, not the cause); cetirizine (malfunction — allergy is the immune system over-reacting); amoxicillin left over from a chest infection (pathogen — bacteria); clotrimazole cream (pathogen — fungus, for ringworm); Coartem (pathogen — the malaria protist). The assistant said amoxicillin works only against bacteria because it stops them building cell walls, and a cold is caused by a virus, which has no cell wall — and added that unused antibiotics should not be kept for next time, which is the point of Lesson 2.4.
Key terms
| Disease | A condition in which part of the body does not work as it should, with signs and symptoms. |
| Non-infectious disease | A disease caused by a malfunction of the body — genes, diet, lifestyle, the immune system, ageing — that cannot be passed on. |
| Infectious (communicable) disease | A disease caused by a pathogen that can be passed from one organism to another. |
| Micro-organism | A living thing too small to see without a microscope: bacteria, single-celled fungi, protists — and, loosely, viruses. |
| Pathogen | A micro-organism that causes disease in its host. |
| Fungus | A eukaryotic organism with a nucleus and a chitin cell wall, growing as yeast cells or threads (hyphae) and reproducing by spores; e.g. the ringworm fungus. |
| Bacterium | A single prokaryotic cell, 0.5–5 μm, with no nucleus, reproducing by binary fission. |
| Virus | A particle of genetic material in a protein coat, 20–300 nm, that reproduces only inside a host cell. |
| Toxin | A poison released by a pathogen that damages the host. |
| Mutation | A copying error in genetic material that is passed on; the source of new variants of a pathogen. |
| Micrometre (μm) / nanometre (nm) | One thousandth / one millionth of a millimetre. |
Think like a scientist
Criterion A Read the passage, then answer.
At a cattle post near Maun, a vet is called because several calves are lame, drooling and have blisters on their tongues and hooves. She diagnoses foot-and-mouth disease and notes that one calf also has an infected hoof wound, swollen and full of pus, which she treats with an antibiotic injection; the wound heals in a week, but the antibiotic makes no difference to the blisters. The herder who has been handling the calves shows her a red, scaly ring on his forearm that is slowly spreading. Two days later his family has diarrhoea and fever after a meal of chicken cooked over a fire that went out early. The herder’s mother, who has type 2 diabetes, was not at the meal and is well.
(a) Define the term pathogen, and identify which illness in the passage is not caused by one. [2]
(b) Name the type of pathogen responsible for the calves’ blisters, the herder’s ring and the family’s diarrhoea, and for each give one structural feature that distinguishes it from the other two. [3]
(c) Explain why the antibiotic cured the hoof wound but had no effect on the blisters. [3]
(d) Compare how a bacterium and a virus reproduce. [2]
(e) The herder’s ring is described as a communicable disease. Explain what this means and describe the most likely way he caught it. [2]
(f) The foot-and-mouth virus is about 25 nm across; a salmonella cell is about 2.5 μm long. Calculate how many virus particles would fit end to end along one salmonella cell, showing your unit conversion. [2]
Model answer
(a) A pathogen is a micro-organism (fungus, bacterium, virus or protist) that causes disease in the organism it infects. The mother’s type 2 diabetes is not caused by a pathogen: it is a malfunction of her own body’s control of blood sugar, and she could not have caught it or passed it on.
(b) The blisters: a virus — not a cell at all, but genetic material inside a protein coat, only 25 nm across. The ring: a fungus — eukaryotic cells with a nucleus and a chitin cell wall, growing as threads (hyphae) through the skin. The diarrhoea: a bacterium (salmonella) — a single prokaryotic cell with no nucleus, about 2 μm, with a cell wall of its own.
(c) The pus-filled wound was a bacterial infection, and antibiotics work by attacking structures that bacterial cells have — their cell wall or their ribosomes — so the bacteria died and the wound healed. The blisters were caused by a virus, which has no cell wall, no ribosomes and no metabolism of its own; it reproduces inside the calf’s cells using the calf’s machinery, so there is nothing for the antibiotic to act on, and the infection ran its course unaffected.
(d) A bacterium reproduces on its own by binary fission: it copies its DNA and splits into two identical cells, as often as every 20 minutes given food and warmth. A virus cannot reproduce by itself: it must enter a host cell and make the cell’s own enzymes and ribosomes assemble thousands of new virus particles, which are released when the cell bursts or buds them off.
(e) Communicable means the disease can be passed from one organism to another — here from an animal to a person. Ringworm fungi live in the skin of calves; the herder handled the calves daily, fungal spores from their coats lodged in his skin, and the fungus grew outwards through the dead outer layer, digesting keratin, to produce the spreading ring.
(f) Convert to the same unit: 2.5 μm = 2.5 × 1000 nm = 2500 nm. Number of particles = 2500 nm ÷ 25 nm = 100 virus particles end to end along one bacterium.
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
Distinguish between an infectious disease and a non-infectious disease, giving one example of each.
Reveal answer
An infectious disease is caused by a pathogen and can be passed from one organism to another, e.g. salmonella food poisoning or ringworm [1]; a non-infectious disease is a malfunction of the body — genetic, dietary, lifestyle or immune — and cannot be caught, e.g. diabetes or scurvy [1].
Give three differences between a bacterium and a virus.
Reveal answer
Any three [1 each]: a bacterium is a living cell, a virus is not a cell; a bacterium is micrometres in size, a virus is nanometres (about a hundredth to a tenth as large); a bacterium has a cell wall, cytoplasm and ribosomes, a virus is only genetic material in a protein coat; a bacterium reproduces by binary fission on its own, a virus reproduces only inside a host cell; a bacterium can be killed by antibiotics, a virus cannot.
A yeast cell is 5 μm across. Express this in nanometres and in millimetres.
Reveal answer
5 μm = 5 × 1000 = 5000 nm [1]; 5 μm = 5 ÷ 1000 = 0.005 mm [1].
Explain how salmonella bacteria cause diarrhoea, and why the main danger of the illness is dehydration.
Reveal answer
The bacteria survive the stomach and multiply by binary fission in the gut, reaching millions within hours [1]; they release toxins that damage and irritate the gut lining so that it secretes water and salts into the gut instead of absorbing them, which is passed as diarrhoea [1]; the body loses more water and salts than it takes in, and in a small child or an elderly person the loss can cause the circulation to fail — so treatment is fluids with salts and sugar, not primarily antibiotics [1].
True or false, with a reason: “A pathogen is any micro-organism.”
Reveal answer
False [1]: a pathogen is a micro-organism that causes disease; the great majority of micro-organisms are harmless or useful, including the bacteria in the gut that digest fibre and make vitamins, and the yeast and bacteria used to make bread, beer and yoghurt [1].
A student says fungi, bacteria and viruses should all be treated with the same medicine “because they are all germs”. Explain, using their structures, why three different kinds of medicine are needed.
Reveal answer
A medicine works by attacking something the pathogen has and the patient does not: antibiotics target the bacterial cell wall or bacterial ribosomes, which fungi and viruses do not have in that form [1]; fungi are eukaryotic cells much like ours, so antifungals must target the few differences, such as the chitin cell wall or the fungal membrane [1]; a virus has no cell, wall, ribosome or metabolism of its own and reproduces inside our cells, so an antiviral must block a specific enzyme of that particular virus — which is why there is no general “germ killer” and why the pathogen must be identified first [1].
Summary
- Disease is the body not working as it should. Non-infectious diseases are malfunctions (diabetes, asthma, cancer); infectious, communicable diseases are caused by pathogens and can be passed on.
- Most micro-organisms are harmless or essential; a pathogen is a micro-organism that causes disease.
- Fungi are eukaryotic cells with chitin walls, reproducing by spores (athlete’s foot, ringworm). Bacteria are prokaryotic cells, 0.5–5 μm, dividing by binary fission (salmonella, MRSA). Viruses are not cells — genetic material in a protein coat, 20–300 nm — and reproduce only inside host cells (HIV, H5N1).
- Bacteria damage by toxins and invasion; viruses hijack and destroy cells; fungi digest tissue such as keratin.
- Rapid multiplication plus copying errors means pathogens mutate and change faster than anything else alive — the reason for resistance, new variants and yearly flu vaccines.
- Antibiotics act only on bacteria. Identify the pathogen before you choose the weapon.
Next time → How pathogens spread: air, water and contact, the doubling arithmetic of an outbreak, and the twenty seconds at the tap that break the chain.