Pathogens and communicable disease · Grade 8
Pathogens are microorganisms that cause infectious (communicable) diseases in animals and plants: viruses, bacteria, fungi and protists. They spread by direct contact, by water, through the air and by vectors. Knowing how a disease spreads tells us how to stop it.
Key facts
- Viruses live and reproduce inside cells, which damages them. Bacteria reproduce rapidly and may release toxins that damage tissues and make us feel ill.
- Viral diseases: measles (fever and a red skin rash; spread in droplets; prevented by vaccination), HIV (a flu-like illness, then damage to the immune system; spread by sexual contact and body fluids; controlled by antiretroviral drugs) and tobacco mosaic virus (discoloured leaves, less photosynthesis).
- Bacterial diseases: salmonella food poisoning (fever, cramps, vomiting and diarrhoea, from undercooked food; poultry are vaccinated) and gonorrhoea (a sexually transmitted disease, once treated with penicillin, but many strains are now resistant).
- Fungal and protist diseases: rose black spot (black spots on leaves; fungicides and removing leaves) and malaria (a protist carried by mosquitoes, which are vectors; repeated fevers).
- The spread of disease can be reduced by hygiene, isolating infected people, destroying vectors and vaccination.
- Bacteria divide by binary fission, as often as every 20 minutes: number = starting number × 2^(number of divisions).
- Antibiotics and antiseptics can be compared by the clear zone around a disc on a plate of bacteria: area = πr². Plates are kept at 25 °C in schools so harmful pathogens are less likely to grow.