A virus can live only inside the cells of its host. Most viruses can only live for a short period of time - from a few minutes to several hours - outside a host cell, though some can live longer.
When viruses have infected a suitable host cell or cells, they take over the cells and replicate themselves - producing new genetic material and protein coats - thousands of times. These are then assembled into new virus particles. The host cell or cells then burst and other nearby cells can be infected with the virus.
Viral infections cannot be treated by antibiotics.
HIV stands for Human Immunodeficiency Virus. This infection is transmitted by body fluids, such as blood, often during unprotected sex, but also through cuts and injecting drugs using shared needles.
Immediately after infection, people often suffer mild flu-like symptoms. These pass and for a period of time infected people might not know they are infected.
Months or years after the infection of the HIV virus, it becomes active and starts to attack the patient's immune system. HIV at this point has become AIDS. AIDS stands for Acquired Immune Deficiency Syndrome.
When a patient has AIDS, their immune system is no longer able to deal with other types of infections or cancer.
There is currently no cure for HIV/AIDS, although treatments are very effective, and patients can live long healthy lives. Currently, infected people are given combinations of antiviral drugs, which can slow the development of the condition.
Measles is a very infectious viral disease that is often caught by young children. It is transmitted through the air in tiny droplets after an infected person sneezes or coughs.
The virus causes a fever and skin rash. Measles is usually a mild disease, but if complications arise, it can be very serious. It can cause infection of the eye, ear, respiratory system and brain, and can cause brain damage. It is serious if caught during pregnancy.
In the UK, most children in developed countries are given vaccines against measles, but this is not the case throughout the whole world.
Gonorrhoea is a sexually transmitted disease (STD) caused by a bacteria. It is a common infection, especially amongst people aged 15-24.
To prevent infection, people can:
Gonorrhoea is treated by penicillin, an antibiotic, but resistant strains of the bacterium are developing. It may soon not be possible to treat the disease.
Salmonella is a genus of bacteria that causes food poisoning.
Salmonella is spread by the presence of the bacterium in food. It can be spread: