Pathogens as disease-causing microorganisms, the four types (bacteria, viruses, fungi and protists), how each makes us ill, and named examples of communicable diseases caused by each type.

What this dot point is asking

AQA wants you to define a pathogen, name the four types of pathogen, explain how bacteria and viruses make us ill, and give the named examples of diseases caused by each type that AQA specifies.

What is a pathogen?

The four types of pathogen

• Bacteria are small living cells that reproduce rapidly inside the body and can produce toxins that damage tissues and make us feel ill. Examples include Salmonella and the bacterium that causes gonorrhoea.
• Viruses are not cells; they are much smaller than bacteria. They reproduce inside living cells, and the host cells are often damaged or destroyed when new viruses are released. Examples include measles, HIV and tobacco mosaic virus.
• Fungi can be single-celled or have a body made of threads called hyphae; they may produce spores that spread to other organisms. An example is rose black spot.
• Protists are single-celled eukaryotic organisms, often spread by a vector (such as a mosquito). An example is the Plasmodium that causes malaria.

How pathogens make us ill

Bacteria and viruses reproduce rapidly once inside the body. Bacteria may release toxins (poisons) that damage cells and tissues, which causes many of the symptoms. Viruses live and reproduce inside cells, and when the new viruses burst out the cell is damaged or destroyed, which causes the symptoms. The body's symptoms (fever, rashes, feeling unwell) are the result of this damage plus the body's own immune response.

Fungi and protists cause disease in their own ways. Fungal pathogens such as rose black spot grow on the leaves of plants, where the purple-black spots reduce the area available for photosynthesis, so the plant grows poorly; the fungus spreads by spores in the air and water. Protist diseases such as malaria are usually spread by a vector: the malarial protist Plasmodium is carried by mosquitoes and reproduces inside the host, causing repeated episodes of fever that can be fatal.

For specific diseases AQA expects you to know the symptoms and treatment. For example, measles causes a fever and a red skin rash and can be fatal, so children are vaccinated against it. HIV initially causes flu-like symptoms then attacks the immune cells; it is controlled with antiretroviral drugs, and if untreated it can develop into AIDS. Salmonella causes fever, stomach cramps, vomiting and diarrhoea from the toxins the bacteria produce.

Try this

Q1. Name the four types of pathogen. [2 marks]

• Cue. Bacteria, viruses, fungi and protists.

Q2. Explain how bacteria make us feel ill. [2 marks]

• Cue. They reproduce rapidly and may produce toxins that damage cells and tissues.