The human immunodeficiency virus (HIV) infects cells of the immune system, destroying or impairing their function. Infection with the virus results in progressive deterioration of the immune system, leading to “immune deficiency”. The immune system is considered deficient when it can no longer fulfil its role of fighting infection and disease. Infections associated with severe immunodeficiency are known as “opportunistic infections”, because they take advantage of a weakened immune system. People injecting drugs and sharing needles or other drug equipment increase risk acquiring or transmitting viral infections because this virus spreads through the blood.