Not all parasites are protective

Summary Successful endoparasites of mammals must outwit the sophisticated immune systems of their hosts that have evolved to detect and destroy/eradicate them. Many species of helminth parasite can directly or indirectly manipulate host immunity: helminth‐derived molecules can suppress or skew activity of many immune cell phenotypes, and mobilization of regulatory cells in response to infection can inhibit immune cell activation. Moreover, many investigations, principally in laboratory‐based rodent–helminth systems, demonstrate that infection with helminths (trematode, cestode or nematode) can ameliorate the severity of concomitant disease that model diabetes, inflammatory bowel disease and multiple sclerosis. Ongoing analyses in these model systems may uncover novel approaches to the management and cure of inflammatory diseases that are major global health issues. However, the potential of environmentally or experimentally (i.e. ‘therapeutically’) acquired infection with helminth parasites to exaggerate the severity immunopathologies in the host should not be overlooked. Here, examples of infection with helminth parasites exacerbating concomitant disease and commentary on possible adverse effects of helminth therapy are provided – the intent is not to undermine the development of helminth therapy, but to illustrate caveats that may need to be considered should helminth therapy be utilized as a treatment for inflammatory disease.