Protozoa are one-celled animals found worldwide in most habitats. Most species are free living, but all higher animals are infected with one or more species of protozoa.
Infections range from asymptomatic to life threatening, depending on the species and strain of the parasite and the resistance of the host.
When protozoa enter the blood stream or tissues they can often survive and replicate because they adapt to the resisting natural host defenses. Protozoa cause chronic and persistent infections, because natural immunity against them is weak and because protozoa have evolved multiple mechanisms for evading and resisting specific immunity.
Since they evolved multiple mechanisms for evading and resisting specific immunity and natural immunity, they cause chronic and persistent infections. They stimulate distinct patterns of immune responses due to diversity in their structural and biochemical properties.