Head lice are common among school-age children and are also seen in people who are debilitated or living in crowded conditions with poor bathing facilities. In the U.S., lice usually do not carry other diseases, but historically lice-borne infections, such as typhus, have caused major epidemics. The scabies parasite does not carry other infections but can cause an intensely itchy rash. Most amoebae are spread through water and food, and these parasites have become less common with improved sanitation. Cryptosporidium, which also spreads through the water supply, has caused several outbreaks in the U.S. recently and appears to be harder to eradicate from the water supply than the amoebae.
Toxoplasmosis is fairly common in humans.
It can live in many animals, but in the U.S. it is most commonly caught from cats. If a pregnant woman catches toxoplasmosis, it can be passed to her unborn child and cause serious disease. Most obstetricians will routinely test for toxoplasmosis with a blood test during pregnancy, and pregnant women are advised to avoid close contact with cats, especially with the litter box. Pneumocystis is a protozoan which is found in humans worldwide. Probably all of us carry pneumocystis from time to time, and our normal immune systems keep it under control.