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Biology · Communicable Diseases

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Biology · AQA Paper 1

Communicable Diseases

Pathogens and the Spread of Disease

Pathogens can be viruses, bacteria, protists or fungi, and they spread by direct contact, by water and by air.

  • Pathogen A microorganism that causes disease.
  • Communicable disease A disease caused by a pathogen that can be spread from one organism to another.
  • Virus A tiny pathogen that can only reproduce inside the cells of its host.
Four panels showing a virus, a bacterium, a protist and a fungus, each with a short description.
Pathogens come in four types.

Viral Diseases

Measles and HIV affect people. Tobacco mosaic virus affects plants. All three viruses damage the cells that they infect.

  • Measles A viral disease with a fever and a red skin rash, spread by droplets.
  • HIV A virus that attacks the immune cells and can lead to AIDS.
  • AIDS The late stage of HIV infection, when the immune system is so damaged that it cannot deal with other infections or cancers.
Three panels showing measles with a rash, HIV attacking a white blood cell, and a leaf with the mosaic pattern of tobacco mosaic virus.
Each virus damages the cells it infects.

Measles and HIV compared

  • Measles Fever and a red skin rash; Spread by droplets from coughs and sneezes; Prevented by vaccination
  • HIV A flu-like illness at first, then attacks immune cells; Spread by sexual contact or exchange of blood and body fluids; Controlled with antiretroviral drugs

Bacterial Diseases

Salmonella is spread by food. Gonorrhoea is spread by sexual contact.

  • Salmonella A bacterium that causes food poisoning.
  • Food poisoning An illness caused by eating food that contains pathogens or their toxins.
  • Gonorrhoea A sexually transmitted disease caused by a bacterium.
A flow from poultry to raw food to an undercooked meal to a person with food poisoning, and a diagram of how gonorrhoea is spread by sexual contact and controlled with condoms and antibiotics.
The two diseases spread in different ways, so they are controlled in different ways.
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Biology · AQA Paper 1

Communicable Diseases continued

Salmonella and gonorrhoea compared

  • Salmonella food poisoning Spread in food or unhygienic kitchens; Fever, cramps, vomiting and diarrhoea; Poultry vaccinated in the UK
  • Gonorrhoea Spread by sexual contact; Thick discharge and pain when urinating; Once treated with penicillin, now many resistant strains

How antibiotic resistance spreads

  • Variation In a large population of bacteria, a few have a gene that makes…
  • Antibiotic use The antibiotic kills the non-resistant bacteria but not the resistant ones.
  • Survival The resistant bacteria survive and reproduce, and pass on the gene.
  • A resistant strain Over time, most of the bacteria in the population are resistant…

Fungal and Protist Diseases

Rose black spot is spread by water and wind. Malaria is spread by a mosquito, which is a vector.

  • Rose black spot A fungal disease of roses that causes purple or black spots on the leaves.
  • Fungicide A chemical that kills fungi.
  • Malaria A disease caused by a protist, which causes recurrent fever and can be fatal.
A cycle showing a mosquito biting an infected person, carrying the protist, biting a new person and passing the protist on, with bed nets and draining water as ways of controlling it.
The mosquito is a vector that carries the malarial protist from person to person.

Rose black spot and malaria compared

  • Rose black spot (fungus) Affects plants (roses); Purple or black spots on leaves; Spread by water and wind
  • Malaria (protist) Affects humans; Recurrent fever, and can be fatal; Spread by the mosquito vector

The malaria cycle

  • Infected person The blood of an infected person contains the protists.
  • Mosquito bites A mosquito feeds on the infected person's blood and takes up the protists.
  • Carried by the vector The protists live in the mosquito, which does not become ill.
  • A new person The mosquito bites a healthy person and passes the protists on.