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 measles. The measles virus is spread by inhalation of droplets from sneezes and coughs.
HIV initially causes a flu-like illness. Unless the virus is controlled with antiretroviral drugs, it attacks the body's immune cells. Late stage HIV infection, or AIDS, 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 or by exchange of body fluids such as blood, which happens when drug users share needles.
Tobacco mosaic virus (TMV) is a widespread plant pathogen that affects many species of plant, including tomatoes. It gives the leaves a distinctive mosaic pattern of discolouration. This affects the growth of the plant because of a lack of photosynthesis.