Some organisms have evolved ways of evading or subverting the body's defenses. The malaria parasite, for example, changes its coat proteins to stay one step ahead of the host's immune cells. Herpes viruses become almost undetectable to lymphocytes. The AIDS virus destroys a subset of T cells that are essential for a successful immune response. Thus the immune system fights off many but not all infections. By learning more about how such pathogens work, new ways may be found to thwart them.