Which organisms are extremely small and use hosts as both home and food supply

The organisms are a type of pathogen called a parasite. Parasites thrive, by hitchiking on a host organism. The host usually provides some sort of protection, and just as well nutrition and either through directly eating from the host (ticks and blood, or Brain eating amoeba and brain tissue), or stealing nutrition from the host's digestion like the Tapeworm. 

Usually most parasite would be considered harmless, as they are merely looking for food and shelter. However this does interfere with our bodily processes which can be dangerous and cause illness.