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B5.2 Pathogens and Disease

GCSE Combined Science · AQA · Chapter B5: Communicable Diseases

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: the four types of pathogen, how bacteria and viruses make us ill, and how pathogens spread. 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
  • Describe the life cycle of a pathogen and explain what makes a pathogen highly infectious
  • 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 air, water, direct contact, food and vectors

Key words

  • Pathogen: A microorganism that causes infectious disease. Pathogens may infect plants or animals.
  • Infectious: Describes a pathogen that is easily passed on, or a person who can pass on a disease.
  • 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, e.g. a mosquito.

Four types of pathogen

Pathogens are microorganisms that cause infectious (communicable) disease in plants and animals. There are four types:

  • Virus; Typical size 20 to 300 nm; Example diseases Measles, HIV (humans); tobacco mosaic virus (plants)
  • Bacterium; Typical size about 1 to 5 µm; Example diseases Salmonella food poisoning, gonorrhoea
  • Fungus; Typical size single cells to large networks of threads; Example diseases Rose black spot (plants); athlete's foot (humans)
  • Protist; Typical size single cells, often 10 to 100 µm; Example diseases Malaria

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

The life cycle of a pathogen

All pathogens follow a simple cycle. Most can survive outside a host for a short time, but they can only reproduce inside (or on) a host.

A highly infectious pathogen passes very easily from one host to another. It may spread through the air (measles virus), reproduce very quickly (norovirus, the "winter vomiting bug") or survive a long time without a host (Staphylococcus bacteria). Reproducing quickly means it can infect more hosts faster.

How bacteria and viruses cause disease

Bacteria are single prokaryotic cells with no nucleus. In the warm, nutrient-rich body they reproduce very rapidly by splitting in two. Many produce toxins (poisons) that damage tissues and make us feel ill. Not all bacteria are harmful: many in the gut help us.

Viruses are not cells. They are a strand of genetic material in a protein coat. A virus can only reproduce inside a living cell: the cell makes copies of the virus until it bursts, releasing new viruses. This cell damage makes us ill.

Worked example: comparing sizes

An electron micrograph shows a bacterium 54 mm long at a magnification of × 18 000. A virus is 120 nm long.
Real size of bacterium = image size ÷ magnification = 54 mm ÷ 18 000 = 0.003 mm = 3 µm.
Virus: 120 nm = 0.12 µm. So the bacterium is 3 ÷ 0.12 = 25 times longer.

Answer: The bacterium is 25 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

  • Air (droplets); How it works Coughs and sneezes release tiny droplets full of pathogens, which other people breathe in.; Example Colds, flu, measles
  • Water; How it works Drinking water contaminated with sewage; water splashing onto plants.; Example Cholera; rose black spot spores
  • Direct contact; How it works Touching, sexual contact or sharing body fluids. In plants, leaves rubbing together.; Example HIV, gonorrhoea; TMV
  • Food; How it works Eating undercooked food, or food prepared in unhygienic conditions.; Example Salmonella; E. coli food poisoning
  • Vectors; How it works Another organism carries the pathogen from host to host.; Example Mosquitoes spread malaria

Key fact: The spread of disease can be reduced by hygiene, isolating infected individuals, destroying vectors and vaccination (see B5.3).

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 is the pathogen. The mosquito carries the malaria protist; the protist is the pathogen.

Revision questions

Try these, then check your answers below each one.

Question 1. The diagram shows a bacterial cell. Which label points to a plasmid? (Choose from: A, B, C, D.)

Answer: C

Working: C is a plasmid, a small ring of DNA. A is the cell wall, B is the main loop of DNA and D is a flagellum.

Question 2. Which type of pathogen is not a cell and can only reproduce inside living cells? (Choose from: Bacterium, Virus, Fungus, Protist.)

Answer: Virus

Working: Viruses are not cells. They take over a host cell, which makes copies of the virus. The cell then bursts.

Question 3. True or false? Bacteria make us feel ill by reproducing rapidly and releasing toxins that damage tissues.

Answer: True

Working: True. Bacteria reproduce rapidly in the warm, nutrient-rich body and many release toxins (poisons) that damage tissues.

Related lessons

Continue in the full course

This lesson is section B5.2 (AGCSC0502) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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