Pathogens
- Pathogens are microorganisms that cause infectious (communicable) disease. They can be viruses, bacteria, protists or fungi.
- Bacteria reproduce rapidly inside the body and may produce toxins that damage tissues and make us feel ill.
- Viruses live and reproduce inside cells, causing cell damage.
- Pathogens spread by direct contact, by water or through the air (for example in droplets from coughs and sneezes).
- Spread can be reduced by good hygiene, destroying vectors, isolating infected people and vaccination.
Viral diseases
- Measles: fever and a red skin rash. It spreads by inhaling droplets from sneezes and coughs. It can be fatal, so young children are vaccinated.
- HIV: at first a flu-like illness. It attacks the body's immune cells; without treatment, late-stage HIV infection (AIDS) leaves the immune system unable to fight other infections. It spreads by sexual contact or exchange of body fluids such as blood, for example when drug users share needles. Antiretroviral drugs control it.
- Tobacco mosaic virus (TMV): infects plants such as tomatoes, giving a mosaic pattern of discoloured leaves. This reduces photosynthesis and growth.
Bacterial diseases
- Salmonella food poisoning: fever, abdominal cramps, vomiting and diarrhoea, caused by the bacteria and the toxins they produce. It spreads through undercooked food or food prepared in unhygienic conditions. Poultry in the UK is vaccinated against Salmonella.
- Gonorrhoea: a sexually transmitted disease causing a thick yellow or green discharge and pain on urinating. It used to be treated with penicillin, but many strains are now resistant. It is controlled by antibiotics and by using condoms.
Fungal and protist diseases
- Rose black spot is a fungal disease: purple or black spots appear on leaves, which turn yellow and drop early, reducing photosynthesis. Spores spread by water or wind. It is treated with fungicides and by removing and destroying affected leaves.
- Malaria is caused by a protist. Its life cycle includes the mosquito, which is a vector. It causes repeated episodes of fever and can be fatal.
- Malaria is controlled by preventing mosquitoes from breeding and by using mosquito nets to avoid being bitten.
Key terms
- Pathogen
- A microorganism that causes disease, such as a virus, bacterium, protist or fungus.
- Communicable disease
- A disease that can spread from one person or organism to another.
- Toxin
- A poison produced by bacteria that damages tissues.
- Vector
- An organism, such as a mosquito, that carries a pathogen from one host to another.
- Measles
- A viral disease causing fever and a red rash, spread by droplets.
- HIV
- A virus that attacks the immune system; without treatment it leads to AIDS.
- Tobacco mosaic virus
- A plant virus that causes a mosaic pattern of discoloured leaves.
- Salmonella
- Bacteria that cause food poisoning.
- Gonorrhoea
- A sexually transmitted bacterial disease.
- Malaria
- A disease caused by a protist and spread by mosquitoes.
- Rose black spot
- A fungal disease of roses causing black spots on leaves.