In developed countries, an enormous change has occurred within the human body over the past century—the loss of parasitic worms. Due to improvements in sanitation, roundworms that have inhabited human intestines (such as the hookworm above) and challenged our immune system for millions of years are no longer a threat. Does the end of this long-term relationship come at a cost? Parasitic worms modulate their host's immune response by inducing the proliferation of regulatory T cells. What role do these cells play in the adaptive immune response? How could this effect on regulatory T cells be linked to the hygiene hypothesis?