 Opportunistic infections are caused by bacteria, mycobacteria, fungi or viruses that do not normally cause infections in people with healthy immune systems. Some of these infections can cause public health concerns, especially in areas with limited access to treatment. People who acquire opportunistic infections usually have diseases that affect their immune systems, such as human immunodeficiency virus HIV, or do not have enough white blood cells to fight the infection. However, some people acquire opportunistic infections even though they have normal amounts of white blood cells and are free from known diseases that harm their immune systems. This study will investigate some of the reasons that otherwise healthy people get opportunistic infections to learn more about why some people are more likely to have them. 