Communicable Diseases and Pathogens

A pathogen is a microorganism that causes communicable disease — a disease that can be passed from one organism to another. Communicable diseases are also called infectious diseases.

It is the pathogen that spreads between hosts. The disease is the result: the harm the pathogen does once it is inside the body. Keeping these two ideas separate is the foundation of this whole topic — you catch a pathogen, and the illness is what that pathogen then causes.

Pathogens fall into four groups, and all four can infect plants as well as animals:

Pathogen type	What it is	Typical size relative to others
Bacteria	Single-celled living organisms	Small
Viruses	Not cells; tiny particles that need a host	Smallest
Protists	Single-celled organisms, often spread by a vector	Larger than bacteria
Fungi	Single-celled (e.g. yeast) or many-celled (e.g. moulds)	Variable

Pathogen: a microorganism — a bacterium, virus, protist or fungus — that causes a communicable (infectious) disease in a plant or animal.

The AQA specification names specific diseases for each pathogen type (such as measles, salmonella and malaria). Those examples are covered in a separate lesson; here the focus is the four pathogen groups themselves and how they behave.

Bacteria are single-celled living organisms. Once inside the body, they can reproduce rapidly — dividing again and again as long as conditions suit them. A small number of bacteria can therefore become a very large population within hours.

The damage comes mainly from what bacteria release. Many bacteria produce toxins — poisons that damage the body's tissues and make us feel ill.

So with a bacterial infection there are two linked problems: the rising number of bacteria, and the toxins they release as they grow.

Key idea: bacteria reproduce rapidly inside the body and produce toxins (poisons) that damage tissues and make us feel ill.

Because bacteria are living cells with their own structures, they can be killed by antibiotics — a point that becomes important when comparing them with viruses on the next slide.

A virus is not a cell. It is a tiny particle that cannot reproduce on its own — it must get inside a host cell to multiply. This single fact explains why viruses damage the body the way they do.

Once inside a cell, a virus takes over the cell's machinery and uses it to make many copies of itself. The cell becomes packed with new virus particles and is damaged or destroyed in the process — often bursting open to release the new viruses, which then invade more cells.

Key idea: viruses live and reproduce inside cells, causing cell damage. The harm to the body comes from this cell damage, not from toxins.

This is the crucial contrast with bacteria. Bacteria mostly harm us with toxins released outside their own cells; viruses harm us by reproducing inside our cells and destroying them.

(Extra context — not required by AQA 8461.) Viruses are sometimes described as "on the boundary of living and non-living" because they cannot reproduce without a host. AQA does not ask you to classify viruses as alive or not.

A frequent exam question asks why antibiotics treat bacterial infections but have no effect on viral ones. The reasoning follows directly from how each pathogen works.

• Antibiotics kill bacteria by attacking structures and processes that bacterial cells have — for example, their cell walls or the way they make proteins. Bacteria are living cells, so there is something for the antibiotic to target.
• Viruses are not cells and do not have these structures. They also hide and reproduce inside the body's own cells, so a drug that killed the virus there would risk damaging the host cell too.

Feature	Bacteria	Viruses
Living cell?	Yes	No (a particle)
How it harms the body	Releases toxins	Reproduces inside cells, damaging them
Reproduces on its own?	Yes (rapidly)	No — only inside a host cell
Killed by antibiotics?	Yes	No

Exam tip: antibiotics work on bacteria because bacteria are cells with targetable structures; viruses are not cells and live inside our own cells, so antibiotics cannot reach or kill them.

The deeper detail of antibiotic resistance and how specific drugs work is developed in a later lesson — for this topic you only need the contrast above.

Pathogens move from an infected host to a new host along a small number of routes. AQA names three main ones, and most prevention methods work by blocking a specific route.

• Direct contact — touching an infected organism, an infected surface, or exchanging body fluids. This includes touching diseased plant material.
• Water — drinking or coming into contact with water carrying the pathogen.
• Air — breathing in pathogens carried in droplets released when an infected person coughs or sneezes.

Route	How the pathogen travels	Everyday example of the route
Direct contact	Skin-to-skin, contaminated surfaces, body fluids	Touching an infected plant's leaves
Water	Pathogen carried in drinking or surface water	Drinking untreated, contaminated water
Air	Droplets from coughs and sneezes are breathed in	A sneeze releasing droplets indoors

Remember: identify the route first. If you know how a pathogen spreads, you can usually work out how to stop it.

Some pathogens — many protists in particular — are carried between hosts by a vector: an organism, such as an insect, that transfers the pathogen without getting the disease itself. Destroying vectors is therefore one way to break the chain of infection.

Once you know a pathogen's route, you can choose a prevention method that blocks it. The specification asks you to describe ways to reduce or prevent the spread of disease — a strong answer also justifies the method chosen.

Method	What it does	Route it blocks
Hygiene	Handwashing, disinfecting surfaces, food handling	Direct contact; water; air
Destroying vectors	Killing or removing the organisms that carry pathogens	Vector-borne spread
Isolation	Keeping infected individuals away from others	Direct contact; air
Vaccination	Makes a population immune so a pathogen cannot spread between hosts	All routes (reduces hosts)

Worked walk-through 1 — a pathogen spreads through the air in droplets from coughs and sneezes. A sensible prevention method is isolation of infected people (and good respiratory hygiene, such as covering coughs). This works because it stops infected droplets reaching new hosts, breaking the airborne route.

Worked walk-through 2 — a plant pathogen spreads by direct contact with diseased leaves and tools. A sensible method is strict hygiene: disinfecting tools and washing hands between plants, plus removing infected plants. This works because it removes the contaminated surfaces the pathogen needs to pass from plant to plant.

Exam tip: a full answer names the method and explains how it blocks that pathogen's route — "isolation, because it stops infected droplets reaching new people" scores better than "isolation" alone.

1. Confusing the pathogen with the disease

The pathogen is the microorganism that spreads; the disease is the harm it causes once inside the host. "The disease spreads through the air" is loose — it is the pathogen that spreads, and the disease results.

2. Saying bacteria damage cells from the inside

That description belongs to viruses. Bacteria reproduce rapidly and mainly harm us by releasing toxins that damage tissues. Viruses are the ones that reproduce inside cells and destroy them.

3. Claiming antibiotics kill viruses

Antibiotics target structures found in bacterial cells. Viruses are not cells and reproduce inside the body's own cells, so antibiotics have no effect on them.

4. Naming a prevention method without justifying it

Marks for prevention questions usually require the reason. State the method and explain which route it blocks — for example, "treating water, because the pathogen spreads through contaminated water".

5. Forgetting that pathogens infect plants too

The four pathogen types can infect plants as well as animals. Routes such as direct contact and prevention methods such as hygiene apply to plant diseases as well.

6. Listing only one route of spread

AQA names three general routes — direct contact, water and air — plus spread by vectors. A question asking "how might this pathogen spread?" often expects more than one possibility.