Communicable diseases — AQA GCSE Combined Science: Trilogy

Revision notes

9 short notes, in the order of the specification. Each one in short:

1. Communicable (infectious) diseases

   A pathogen is a microorganism that causes infectious disease, and may be a virus, bacterium, protist or fungus. Communicable diseases are spread by direct contact, by water or by air. Bacteria and viruses may reproduce rapidly inside the body. Bacteria may produce toxins that damage tissues, and viruses live and reproduce inside cells, causing cell damage.

2. Viral diseases

   Measles, HIV and tobacco mosaic virus are viral diseases. Measles causes a fever and a red skin rash, is spread by inhaling droplets from sneezes and coughs, and is prevented by vaccination. HIV attacks immune cells unless controlled with antiretroviral drugs, leading to AIDS. TMV gives a mosaic pattern on leaves, reducing photosynthesis and growth.

3. Bacterial diseases

   Salmonella food poisoning and gonorrhoea are bacterial diseases. Salmonella is spread in food, and its symptoms are caused by the bacteria and the toxins they secrete; UK poultry are vaccinated against it. Gonorrhoea is a sexually transmitted disease that was easily treated with penicillin until resistant strains appeared. It is controlled with antibiotics and condoms.

4. Fungal diseases

   Rose black spot is a fungal disease of plants that causes purple or black spots on the leaves, which often turn yellow and drop early. This reduces photosynthesis, so less glucose is made and the growth of the plant is affected. It is spread by water or wind, and is treated with fungicides and by removing and destroying affected leaves.

5. Protist diseases

   Malaria is a disease caused by a protist, whose life cycle includes the mosquito. It causes recurrent episodes of fever and can be fatal. The mosquito is a vector: it carries the protist from one person to another when it feeds on blood. Malaria is controlled by stopping mosquitos from breeding and by using mosquito nets to avoid bites.

6. Human defence systems

   Human defence systems stop pathogens entering the body or destroy them. Non-specific defences include the skin as a barrier, hairs and mucus in the nose, mucus and cilia in the trachea and bronchi, and hydrochloric acid in the stomach. If pathogens get in, white blood cells destroy them by phagocytosis, antibody production and antitoxin production.

7. Vaccination

   Vaccination is introducing small quantities of dead or inactive forms of a pathogen into the body to stimulate white blood cells to produce antibodies. If the same pathogen re-enters the body, the white blood cells respond quickly with the correct antibodies, preventing infection. If a large proportion of the population is immunised, the pathogen cannot spread easily.

8. Antibiotics and painkillers

   Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body. Specific bacteria should be treated with specific antibiotics. Antibiotics cannot kill viruses, and the emergence of strains resistant to antibiotics is of great concern. Painkillers and other medicines treat the symptoms of disease but do not kill pathogens.

9. Discovery and development of drugs

   Drugs were traditionally extracted from plants and microorganisms, such as digitalis from foxgloves and aspirin from willow, but most new drugs are now synthesised by chemists. New drugs are tested for toxicity, efficacy and dose: first in preclinical testing on cells, tissues and live animals, then in clinical trials on healthy volunteers and patients.