Certain types of viruses can persist in a latent or inactive form in the body, and in normal healthy people, they are controlled by the immune system.
However, these viruses can become reactivated in people whose immune systems have been compromised by disease or medications, such as patients with HIV-AIDS or transplant recipients receiving immunosuppressive therapy. This can cause a variety of serious and potentially life-threatening illnesses, including several types of cancer. In addition, persistent viral infections are involved in the development of cancer in people with apparently normal immune systems. For example, Epstein Barr virus is implicated in the development of nasopharyngeal cancer in East Asia and Burkitt's lymphoma in certain regions of Africa.