Infections are by far the most important causes of disease in Africa. The immune system has evolved to protect us from the multitude of microorganisms and parasites to which we are exposed every day (Chaplin 2010). It has two main components. The first is the non-specific (or innate) immune system, which acts independently of any previous exposure to a pathogen, and includes cells such as phagocytes and neutrophils. The second is the specific (or adaptive) immune system, which responds to individual pathogens and has a memory process, so that subsequent responses are more effective with repeated exposures. When the immune response is most effective, the pathogen is eliminated with little or no damage to the human host and the host retains a memory response, which prevents future episodes of disease. The immune system is usually extremely beneficial. However, some components cause adverse effects, which we see as disease. These include fever, inflammation and tissue destruction. Occasionally, abnormal immune responses cause disease directly. This is the case in diseases such as rheumatoid arthritis and asthma.