Schistosomes wed for life—sometimes several decades—and literally live together. The female (the smaller of the two shown at left) snuggles into a special groove on the body of her mate. Schistosomes swimming among bathers have wormed their way into 200 million humans, causing debilitating fevers and other maladies. Dwelling in blood vessels, the parasites evade the immune system by cloaking themselves with pilfered molecules. AVERAGE SIZE: 0.5 inch (13 millimeters).