Human infections come from different types of pathogen. Measles is viral: it spreads in droplets and causes fever and a red skin rash. Athlete's foot is fungal and causes itchy, cracked skin between the toes. Salmonella food poisoning is bacterial: it is caught from contaminated food and causes fever, stomach cramps, vomiting and diarrhoea.
Sexually transmitted infections (STIs) are spread by sexual contact. HIV also spreads through blood and from a mother to her baby, and it can lead to AIDS. Antiretroviral drugs control HIV but do not cure it. Chlamydia is a bacterial STI that often has no symptoms, so it can spread without being noticed. It is treated with antibiotics. Gonorrhoea is also bacterial. Condoms reduce the spread of STIs.
Three plant diseases are named in the specification. Tobacco mosaic virus (TMV) causes a mosaic of discoloured patches on leaves, so less photosynthesis takes place and the plant grows poorly. Barley powdery mildew is caused by a fungus, Erysiphe graminis, which forms white powdery patches on leaves. Crown gall disease is caused by the bacterium Agrobacterium tumefaciens and produces tumour-like growths on stems and roots.