In 1892 the Russian scientist Dmitri Ivanovsky was studying tobacco mosaic disease, which blotched the leaves of tobacco crops. He squeezed sap from infected leaves and pushed it through a porcelain filter fine enough to trap bacteria. The filtered sap still infected healthy plants. Ivanovsky suggested the cause might be a toxin made by bacteria, or bacteria small enough to slip through the filter. In 1898 Martinus Beijerinck repeated the work and argued it was a new kind of infectious agent. We now call it a virus, and tobacco mosaic virus is one of the pathogens on your specification. Nobody saw it until electron microscopes were used in the late 1930s.
This lesson covers communicable diseases for AQA GCSE Biology Topic 3 (Infection and response). It's also useful for Combined Science.
Pathogens and how they spread
A communicable (infectious) disease is caused by a pathogen and can pass from one host to another. Pathogens are microorganisms: viruses, bacteria, protists and fungi. They can infect plants as well as animals, and they spread by direct contact, by water or by air.
- Bacteria reproduce rapidly and may produce toxins that damage tissues.
- Viruses live and reproduce inside cells, damaging them.
You can reduce spread with simple hygiene, by destroying or controlling vectors, by isolating infected individuals, and by vaccination.
Viral diseases
- Measles: fever and a red skin rash. It is serious and can be fatal. It spreads when people inhale droplets from sneezes and coughs, and most young children are vaccinated against it.
- HIV: at first a flu-like illness. Unless it is controlled with antiretroviral drugs, the virus attacks the body's immune cells. Late stage HIV infection, or AIDS, is when the immune system is so damaged it can't deal with other infections or cancers. HIV spreads by sexual contact or by exchange of body fluids such as blood, for example when drug users share needles.
- Tobacco mosaic virus (TMV): infects many plant species, including tomatoes. It gives a mosaic pattern of discolouration on leaves, so the plant photosynthesises less and grows less.
Bacterial diseases
- Salmonella food poisoning: caught from food containing the bacteria or food prepared in unhygienic conditions. Fever, abdominal cramps, vomiting and diarrhoea come from the bacteria and the toxins they secrete. In the UK, poultry are vaccinated against Salmonella.
- Gonorrhoea: a sexually transmitted disease with a thick yellow or green discharge and pain on urinating. It used to be easily treated with penicillin, but many resistant strains have appeared. Antibiotic treatment and condoms reduce its spread.
Fungal disease
Rose black spot: purple or black spots on the leaves, which often turn yellow and drop early. Less leaf means less photosynthesis, so the plant grows less well. The spores spread by water or wind. Gardeners use fungicides and/or remove and destroy the affected leaves.
Protist disease
Malaria: the pathogen is a protist, and its life cycle includes the mosquito, which acts as a vector. In the 1890s Ronald Ross showed that mosquitoes carry the malaria parasite. Malaria causes recurrent episodes of fever and can be fatal. Stopping mosquitoes breeding and sleeping under mosquito nets both cut its spread.
Common exam mistakes
- "Mosquitoes cause malaria." A protist causes it; the mosquito is the vector.
- "Rose black spot is a virus." It's a fungus, so it spreads by spores and is treated with fungicides.
- "Antibiotics treat measles or HIV." Both are viruses. HIV is controlled with antiretroviral drugs.
- "TMV kills the plant with toxins." Say that discoloured leaves photosynthesise less, so growth slows.
- Vague control answers. Match the method to the spread: vaccination for measles, nets for malaria, hygiene for Salmonella.
Frequently asked questions
What are the four types of pathogen in GCSE Biology?
Viruses, bacteria, protists and fungi. Each one causes at least one disease you need to know.
Is malaria caused by a virus or a bacterium?
Neither. Malaria is caused by a protist, which mosquitoes carry from person to person.
How do bacteria and viruses make you ill?
Bacteria may release toxins that damage tissues. Viruses reproduce inside your cells and damage them.
How is the spread of communicable diseases reduced?
By hygiene, controlling vectors, isolating infected people and vaccination.
Try it
Say each answer before you flip the card, then sort the seven diseases into the four pathogen types from memory. Then put yourself in Ivanovsky's place: the sap still infects plants after filtering, so the cause isn't an ordinary bacterium. What would you have tested next?
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