Measles is a viral disease. The symptoms are a fever and a red skin rash. It is a serious illness that can be fatal if complications arise, so most young children are vaccinated against it. The measles virus is spread when people breathe in droplets from the sneezes and coughs of an infected person.
HIV first causes a flu-like illness. If it is not successfully controlled with antiretroviral drugs, the virus attacks the body's immune cells. Late stage HIV infection is called AIDS. It happens when the immune system is so badly damaged that it can no longer deal with other infections or cancers. HIV is spread by sexual contact and by the exchange of body fluids such as blood, for example when drug users share needles.
Tobacco mosaic virus (TMV) is a widespread plant pathogen that affects many species of plant, including tomatoes. It gives leaves a distinctive mosaic pattern of discolouration. Growth is affected because the plant carries out less photosynthesis.