Viral, Bacterial, Fungal and Protist Diseases

A pathogen is a microorganism that causes a communicable disease. AQA names seven specific diseases you must know, grouped by the type of pathogen that causes each one.

For every disease, learn four things: the pathogen type, the symptoms, how it spreads, and how it is treated or prevented. The table below is your revision map for the whole topic.

Disease	Pathogen type	How it spreads	Key symptoms	Treatment / prevention
Measles	Virus	Inhaling droplets from coughs/sneezes	Fever and a red skin rash	Vaccination of young children
HIV	Virus	Sexual contact; exchange of body fluids	Early flu-like illness; later immune damage	Antiretroviral drugs
Tobacco mosaic virus (TMV)	Virus	Contact between plants; via the environment	Mosaic discolouration on leaves	No cure; remove infected plants (control not named by AQA)
Salmonella	Bacterium	Ingesting contaminated/unhygienic food	Fever, cramps, vomiting, diarrhoea	Vaccinate poultry; food hygiene
Gonorrhoea	Bacterium	Sexual contact	Yellow/green discharge; pain on urinating	Antibiotics; barrier contraception (condom)
Rose black spot	Fungus	Water and wind in the environment	Purple/black spots on leaves; leaves drop	Fungicides; remove and destroy affected leaves
Malaria	Protist	Mosquito vector (a biting insect)	Recurrent episodes of fever	Stop mosquitoes breeding; mosquito nets

Whenever a question describes a disease, your first job is to identify the pathogen type. That single decision rules in or out every treatment — antibiotics work on bacteria, not viruses, and vaccines target specific pathogens.

The following slides take each disease in turn and add the detail behind this table.

Measles is caused by a virus. Its two main symptoms are a fever and a red skin rash.

Measles is a serious illness. If complications develop it can be fatal, which is why most young children are vaccinated against it. The vaccine does not treat measles once a child has it — it prevents infection in the first place by preparing the immune system in advance.

How it spreads: measles is passed on by inhalation of droplets from an infected person's sneezes and coughs. Tiny droplets carrying the virus hang in the air and are breathed in by people nearby.

Measles control: there is no cure once infected, so prevention relies on vaccination. Widespread vaccination also protects unvaccinated people by reducing how easily the virus can spread through a population.

Because the virus travels in airborne droplets, crowded indoor spaces make transmission easier. This is the same droplet route by which many other respiratory infections spread, so the prevention logic — vaccinate, and limit close contact with infected people — is one you can reuse across the topic.

HIV is caused by a virus. It initially causes a mild, flu-like illness. After this early stage a person may feel well for years, but the virus is still active.

Unless it is successfully controlled with antiretroviral drugs, HIV attacks the body's immune cells. Over time this damages the immune system. Late-stage HIV infection is called AIDS: the immune system becomes so badly damaged that the body can no longer deal with other infections or with cancers.

How it spreads: HIV is passed on by sexual contact or by the exchange of body fluids such as blood. A common route is drug users sharing needles, which transfers infected blood directly from one person to another.

HIV vs AIDS: HIV is the virus; AIDS is the late stage of infection where the immune system has failed. A person can carry HIV for a long time before — and without ever reaching — the AIDS stage if treatment controls the virus.

Antiretroviral drugs do not remove the virus from the body, but they keep its levels low enough to protect the immune cells. This is control, not a cure — which is why prevention by avoiding fluid exchange (for example, not sharing needles) still matters.

Not every pathogen infects humans. Tobacco mosaic virus (TMV) is caused by a virus and is a widespread plant pathogen. It affects many species, including tomato plants.

The symptom is a distinctive 'mosaic' pattern of discolouration on the leaves — patches of pale, mottled colour against the normal green. This is not just a cosmetic problem.

Why it harms the plant: the discoloured areas contain less of the green pigment needed to absorb light, so they carry out less photosynthesis. Photosynthesis is how a plant makes its food, so reduced photosynthesis means the plant has less energy and its growth is affected.

TMV in one line: a plant virus that gives leaves a mosaic discolouration, lowering photosynthesis and stunting growth.

TMV is spread by contact — for example, on a gardener's hands or tools moving between plants, or between leaves that touch.

(Extra context — beyond AQA 8461: the spec names TMV's symptoms but does not specify a control method. In practice there is no cure, so growers remove infected plants and use good field hygiene to limit spread.)

(Extra context, beyond AQA 8461: the green pigment responsible for absorbing light for photosynthesis is chlorophyll, found in chloroplasts — useful for explaining why the pale mosaic areas photosynthesise less.)

Two bacterial diseases are named by AQA. Both are caused by bacteria, and bacterial diseases can be treated with antibiotics — a key contrast with the viral diseases above.

Salmonella food poisoning is caused by Salmonella bacteria. It is spread by bacteria ingested in food, or on food prepared in unhygienic conditions. The symptoms — fever, abdominal cramps, vomiting and diarrhoea — are caused both by the bacteria themselves and by the toxins they secrete.

To control the spread, poultry in the UK are vaccinated against Salmonella so that the bacteria are far less likely to enter the food chain. Good food hygiene — thorough cooking and avoiding cross-contamination — reduces infection further.

Gonorrhoea is a sexually transmitted disease (STD) caused by a bacterium. Its symptoms are a thick yellow or green discharge from the vagina or penis and pain on urinating. It is spread by sexual contact.

Disease	Spread	How it is controlled
Salmonella	Ingested in/on contaminated food	Vaccinate poultry; food hygiene
Gonorrhoea	Sexual contact	Antibiotics; barrier contraception such as a condom

Gonorrhoea and resistance: it was once easily treated with the antibiotic penicillin, but many resistant strains have since appeared. Spread is now controlled by treatment with antibiotics or by using a barrier method of contraception such as a condom.

Rose black spot is caused by a fungus. Like TMV, it is a plant disease — this time affecting rose plants.

The symptoms are purple or black spots that develop on the leaves. The affected leaves often turn yellow and drop early.

Why it harms the plant: with fewer healthy leaves, the plant carries out less photosynthesis, so its growth is affected — the same cause-and-effect you met with TMV, even though the pathogen is completely different.

How it spreads: rose black spot is spread in the environment by water or wind, which carry the fungal spores from plant to plant.

Rose black spot control: treat with fungicides (chemicals that kill fungi) and/or remove and destroy the affected leaves so the fungus cannot keep spreading from them.

Notice the pattern across the two plant diseases: both TMV (a virus) and rose black spot (a fungus) reduce photosynthesis and stunt growth, but only the fungal disease can be treated with a fungicide. Matching the treatment to the pathogen type is exactly what exam questions test.

The pathogens that cause malaria are protists. Malaria causes recurrent episodes of fever and can be fatal.

What makes malaria distinctive is its life cycle, which includes the mosquito. The mosquito is a vector — an organism that carries a pathogen from one host to another without getting the disease itself. The malarial protist is passed to people when an infected mosquito bites them.

Because the mosquito is essential to the spread, malaria is controlled by attacking the vector rather than only treating sick people:

• Preventing mosquitoes from breeding — for example, removing or draining the standing water where mosquito larvae develop.
• Using mosquito nets — sleeping under a net stops infected mosquitoes biting people at night.

Vector logic: break the mosquito's role and you break the chain of infection. This is why malaria control targets the insect, not just the patient.

Applied walk-through 1 — identify the disease and a control measure. A patient has recurrent episodes of fever and lives in a region with many mosquitoes near standing water.

1. The mosquito link and recurrent fever point to malaria.
2. Malaria is caused by a protist, spread by a mosquito vector.
3. A valid control measure: drain the standing water to stop mosquitoes breeding, and use mosquito nets to avoid bites.

Applied walk-through 2 — match symptoms to pathogen and prevention. A patient reports a thick yellow discharge and pain on urinating, following sexual contact.

1. Sexual transmission plus that discharge identify gonorrhoea.
2. It is caused by a bacterium, so antibiotics can treat it (though resistant strains exist).
3. A valid prevention measure: a barrier method of contraception such as a condom.

1. Treating a virus with antibiotics

Antibiotics kill bacteria, so they help with Salmonella and gonorrhoea but do nothing for measles, HIV or TMV. If the pathogen is a virus, reach for vaccination or antiviral control instead.

2. Confusing HIV with AIDS

HIV is the virus. AIDS is the late stage of infection, when the immune system is so damaged it can no longer fight other infections or cancers. Controlling HIV with antiretroviral drugs can stop a person ever reaching the AIDS stage.

3. Saying TMV or rose black spot infects humans

Both are plant diseases. TMV (a virus) and rose black spot (a fungus) damage leaves and reduce photosynthesis — they do not cause illness in people.

4. Forgetting the mosquito is a vector, not the pathogen

In malaria the protist is the pathogen that causes the disease; the mosquito only carries it between hosts. Control works by targeting the mosquito's breeding and biting.

5. Giving a treatment that does not match the pathogen

Match the control to the cause: vaccinate young children against measles and poultry against Salmonella; use antiretrovirals for HIV; use fungicides for rose black spot; use nets and breeding control for malaria. A mismatched answer loses the mark even if the disease is named correctly.

6. Vague transmission answers

"It spreads through contact" is rarely enough. Be specific: measles by inhaling droplets, HIV by body fluids such as blood (including shared needles), Salmonella by contaminated food, gonorrhoea by sexual contact, rose black spot by water and wind, and malaria by a mosquito bite.