Figure 1.

Categories of parasites, using the broader definition to include fungi and ectoparasites. An ectoparasite (a louse of mice) has recently been found to exert similar immunomodulatory effects to those associated with gastrointestinal helminth infection in a wild wood mouse population [2]. Shown are, clockwise from top left, Leishmania mexicana protozoal promastigotes (L. Prieto-Lafuente); Candida albicans (from http://www.dreamstime.com/stock-photo-of-candida-abicans-image894348 webcite); Ixodes hexagonus (from Wikipedia [15]); and Heligmosomoides polygyrus adult nematode worms (C Finney).