HIV (human immuno-deficiency virus) is a virus that affects the t-cells in the body (t-cells help fight infections). The t-cells are used to produce more HIV cells then the t-cell breaks down. Because there are less cells to fight infection the body has a lower immune system and so can be overcome by infections more easily (leading to AIDS). HIV is caused by contact with an infected persons bodily fluids. This can be through unprotected sex (including anal sex), needle sharing or through transfusion of infected blood.