Plasmodium falciparum is a protist parasite that causes the most dangerous form of malaria, infecting red blood cells and causing symptoms like fever, vomiting, and headaches. It is transmitted by mosquitoes and can be fatal if untreated. Entamoeba histolytica is also a protist parasite that causes amoebic dysentery through cysts in contaminated water or food. Symptoms include bloody diarrhea and it can be deadly. Human parvovirus B19 is a single-stranded DNA virus that causes fifth disease, spreading through respiratory droplets. It presents as a facial rash in children and flu-like symptoms, while pregnant women are at risk of miscarriage.