5.2 Pathogens and Disease

A free GCSE Biology lesson for AQA (8461) from OBA School: the four types of pathogen, how bacteria and viruses make us ill, and how diseases spread in animals and plants. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Name the four types of pathogen and give an example of a disease caused by each
• Explain how bacteria and viruses cause disease
• Compare the sizes of bacteria and viruses, using magnification and units (µm and nm)
• Describe how pathogens spread by direct contact, water, air, food and vectors in animals and plants

Key words

• Pathogen: A microorganism that causes infectious disease. Pathogens may infect plants or animals.
• Bacteria: Very small single-celled organisms (prokaryotes). Some are pathogens. Singular: bacterium.
• Virus: A tiny particle made of genetic material inside a protein coat. It can only reproduce inside living cells.
• Toxin: A poison. Some bacteria release toxins that damage tissues and make us feel ill.
• Vector: An organism that carries a pathogen from one host to another without getting the disease, e.g. a mosquito.
• Host: The organism that a pathogen lives in or on.

Four types of pathogen

Pathogens are microorganisms that cause infectious (communicable) disease. They may infect plants or animals. There are four types:

• Virus; Typical size 20 to 300 nm; Human disease Measles, HIV, influenza; Plant disease Tobacco mosaic virus (TMV)
• Bacterium; Typical size about 1 to 5 µm; Human disease Salmonella food poisoning, gonorrhoea; Plant disease Crown gall disease
• Fungus; Typical size single cells to large networks of hyphae; Human disease Athlete's foot; Plant disease Rose black spot
• Protist; Typical size single cells, often 10 to 100 µm; Human disease Malaria; Plant disease Potato blight (caused by a protist-like water mould)

Key fact: 1 mm = 1000 µm (micrometres) and 1 µm = 1000 nm (nanometres). Bacteria are measured in µm, viruses in nm.

How bacteria cause disease

Bacteria are single prokaryotic cells. They have no nucleus. Inside the body, in warm conditions with plenty of nutrients, they reproduce very rapidly by splitting in two. Some bacteria produce toxins (poisons) that damage tissues and make us feel ill.

How viruses cause disease

Viruses are not cells. They are much smaller than bacteria. A virus can only reproduce inside a living cell. It takes over the cell, which makes many copies of the virus. The cell then bursts and the new viruses infect more cells. This cell damage makes us ill.

Worked example: comparing sizes

An electron micrograph shows a bacterium 36 mm long at a magnification of × 12 000. A virus is 150 nm long.
Real size of bacterium = image size ÷ magnification = 36 mm ÷ 12 000 = 0.003 mm = 3 µm.
Virus: 150 nm = 0.15 µm. So the bacterium is 3 ÷ 0.15 = 20 times longer.

Answer: The bacterium is 20 times longer than the virus

Exam tip: A light microscope cannot resolve objects smaller than about 0.2 µm, so most viruses can only be seen with an electron microscope.

How pathogens spread

Communicable diseases spread when pathogens pass from one host to another.

• Air (droplets); How it works Coughs and sneezes release droplets full of pathogens. Others breathe them in.; Example Influenza, measles
• Water; How it works Drinking water contaminated with sewage, or water splashing onto plants.; Example Cholera; rose black spot spores in rain
• Direct contact; How it works Touching, sexual contact or exchanging body fluids. In plants, leaves rubbing together.; Example HIV, gonorrhoea; TMV
• Food; How it works Eating food that contains pathogens or was prepared in unhygienic conditions.; Example Salmonella food poisoning
• Vectors; How it works An animal carries the pathogen from host to host.; Example Mosquitoes spread malaria

Plant pathogens can also spread through soil, on dirty tools, and by insects such as aphids that feed on plant sap.

Key fact: Pathogens spread fastest when people (or plants) are crowded close together, so overcrowding, poor hygiene and dirty water all increase the spread of disease.

Common mistakes

• Calling viruses cells. They are particles that can only reproduce inside cells.
• Saying bacteria make us ill "by being there". Say they reproduce rapidly and release toxins that damage tissues.
• Mixing up units: 1 µm = 1000 nm, not 100 nm.
• Dividing the wrong way: real size = image size ÷ magnification. Convert mm to µm by multiplying by 1000.
• Thinking a vector gets the disease. A mosquito carries the malaria protist but is not made ill by it.

Revision questions

Try these, then check your answers below each one.

Question 1. Which type of pathogen is usually the smallest? (Choose from: Bacterium, Virus, Fungus, Protist.)

Answer: Virus

Working: Viruses (about 20–300 nm) are much smaller than bacteria (about 1–5 µm). They are not cells.

Question 2. True or false? Viruses can reproduce outside living cells.

Answer: False

Working: False. Viruses can only reproduce inside living cells. The cells are damaged when new viruses are released.

Question 3. How do bacteria make us feel ill? (Choose from: They live inside our cells and burst them, They reproduce rapidly and release toxins that damage tissues, They eat our red blood cells, They block the airways.)

Answer: They reproduce rapidly and release toxins that damage tissues

Working: Bacteria reproduce rapidly in the body. Some produce toxins (poisons) that damage tissues and make us feel ill. Bursting cells is how viruses cause damage.

Related lessons

• Previous lesson: 5.1 Health and Disease
• Next lesson: 5.3 Growing Bacteria in the Lab
• In this chapter: 5.4 Preventing Bacterial Growth
• In this chapter: 5.5 Preventing Infections
• In this chapter: 5.6 Viral Diseases

Continue in the full course

This lesson is section 5.2 (AGCBH0502) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.