Some harmful protists include plasmodium, gregarines, haemosporidia and coccidians. Others include babesia, toxoplasma, adeleorina and some forms of amoeba. Some of these protists are parasites on invertebrates such as gastropods while others are parasites on vertebrates such as humans.
Plasmodium is a protist that causes malaria. The organism is passed from one host to another either by a mosquito or, more rarely, through sharing of contaminated needles. This happens because the parasite needs to spend part of its life cycle inside the mosquito and part of its life cycle inside a vertebrate. The sickness is caused because the organism invades the host's red blood cells and deforms them. This is why sickle cell anemia, where the red blood cells are already deformed, offers some protection against malaria.
Coccidians and gregarines live in the guts of their hosts, but coccidians parasitize vertebrates, while gregarines are found in mollusks and types of arthropods. Both of these protists also have complex life cycles that require a host to complete. Coccidians cause diseases such as cryptosporidiosis and cyclosporiasis.
The entamoeba histolytica is responsible for amoebic dysentery. They also live in the gut and destroy the bacteria that live there. This causes symptoms such as constipation that alternates with bloody, watery diarrhea, flatulence and abdominal cramps.