Viral Disease

Measles is a viral disease. It is spread by through droplets from an infected person's sneeze or a cough. People with measles develop a red skin rash, and they'll show signs of a fever (a high temperature). Measles can be very serious or even fatal if there are complications. For example, measles can sometimes lead to pneumonia (a lung infection) or a brain infection called encephalitis. Most people are vaccinated against measles when they're young. 

HIV is a virus spread by sexual contact, or by exchanging bodily fluids such as blood. This can happen when people share needles when taking drugs. HIV initially causes flu-like symptoms for a few weeks. Usually, the person doesn’t then experience any symptoms for several years. During this time, HIV can be controlled with antiretroviral drugs. These stop the virus replicating in the body. The virus attacks the immune cells (cells of the immune system — see page 136). If the body's immune system is badly damaged, it can't cope with other infections or cancers. At this stage, the virus is known as late-stage HIV infection or AIDS.

Tobacco mosaic virus: (TMV) is a virus that affects many species of plants, e.g. tomatoes. It causes a mosaic pattern on the leaves of the plants — parts of the leaves become discolored. The discoloration means the plant can't carry out photosynthesis as well, so the virus affects growth.