Although parasites can be harmful, do some actually play a role in sustaining human health? According to the “hygiene hypothesis,” people with autoimmune diseases (an estimated 23.5 million and growing, according to the National Institutes of Health) often grew up in urban areas, where they weren’t exposed to common infectious agents. Indeed, studies have found that parasites such as the hookworm had protective effects against celiac, Crohn’s disease, and other painful intestinal ailments. Taking the chlorophyll-rich, single-celled algae Chlorella pyrenoidosa may offer immune-system regulation benefits and promotes the growth of beneficial bacteria, says the author. Initial research also suggests that Chlorella may alter the expression of the PTEN gene, a commonly mutated gene implicated in several cancers and perhaps autism, says the author.