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 offers an answer while reinforcing an adage: safety in numbers. The researchers have concluded that when large numbers of worms stake out intestinal territory in a host, they generally manage to hang in there for the long term. By contrast, the model suggests that lone worms -- or small groups of them -- get overwhelmed by the immune system and quickly flushed from the gastrointestinal tract. The team's model can simulate host-parasite interactions by accounting for multiple variables: number of parasites, availability of food, body mass of the host, how much of that mass is accessible to parasites versus the immune system. Crucially, it also factors in that parasitic worms can manipulate a host's ability to digest food and mount an immune response.
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