Immunology was founded by studying the body's response to infectious microorganisms, and yet microbial prokaryotes only tell half the story of the immune system. Eukaryotic pathogens--protozoa, helminths, fungi and ectoparasites--have all been powerful selective forces for immune evolution. Often, as with lethal protozoal parasites, the focus has been on acute infections and the inflammatory responses they evoke. Long-lived parasites such as the helminths, however, are more remarkable for their ability to downregulate host immunity, protecting themselves from elimination and minimizing severe pathology in the host.