"As hosting gigs go, it's a tough crowd. More than 1 billion people are host to parasitic worms that take up residence in their intestines. For most, it's a short stay, with the immune system evicting the worms in days or weeks and leaving no trace that the parasites were ever there. In a small percentage of people and other animals, though, the worms gain a permanent mouth-hold and can stick around the intestines for years. That persistence often leads to malnutrition, which in turn tightens the grip of infection, initiating a spiral that becomes difficult to escape. The question of why that small percentage struggles to expel parasites has stymied researchers for a while. But a new mathematical model developed at the University of Nebraska-Lincoln and Royal Netherlands Institute for Sea Research offers an answer while reinforcing an adage: safety in numbers. Clay Cressler, Anieke van Leeuwen and their colleagues have concluded that when large numbers of worms stake...