Parasites are organisms that exist on or in another organism and survive by feeding off the host. According to the Centers for Disease Control and Prevention, the three main types of parasites that cause disease in humans are single-celled protozoa and multi-cellular helminths and ectoparasites.
Protozoa can exist as independent beings or as parasites on host organisms. Protozoa can create serious infections for humans because of their ability to divide and create many organisms from just one original organism.
Helminths are visible to the human eye. These also can be independent beings or parasites. However, they cannot multiply in humans. The three types of parasitic helminths include flatworms, thorny-headed worms and roundworms.
Ectoparasites dig into the skin and remain there for awhile. While this term can apply to mosquitoes, it more specifically refers to organisms like ticks, lice and fleas. Their main danger to humans is through transmission of diseases.