Protozoa are a diverse group of parasites, but malaria, leishmaniasis, and trypanosomiasis globally account for most of the problems encountered in parasitic diseases. The balance between host and parasite is two fold. The parasite may be too virulent for the host or may evade the immune surveillance and thus kills the host. Conversely, the immune response may be vigorous and kill the parasite, thereby jeopardizing its survival. Thus, the survival of any parasite depends on a balance between induction of immunity and escape from surveillance.