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The immune system

When your immune system is healthy, it protects your body against infections caused by bacteria and viruses. White blood cells find and destroy germs that your body sees as 'foreign' and this stops you developing serious diseases.

HIV avoids being destroyed by your immune system by changing its outer 'coat' again and again. It multiplies (replicates) inside a type of white blood cell called a CD4 cell. This kind of cell is normally involved in helping other types of immune cell to attack and destroy bacteria and viruses.

As HIV multiplies, it destroys the CD4 cells in your body, so there are fewer of them. Having fewer CD4 cells means that your body's ability to fight other infections is weakened and your defences against certain cancers are reduced