The kakapo, a large flightless parrot that can live 95 years and perhaps longer, is dangerously close to extinction. Once found throughout New Zealand, the population has dwindled to fewer than 150.

Conservation biologists are doing everything they can to keep the kakapo from vanishing. And so, when they discovered a few years ago that a pair of captive kakapos were infected with tapeworms, they did the obvious thing: They dewormed the birds.

Hamish G. Spencer, a geneticist at the University of Otago in New Zealand, thinks that was unwise. If endangered species are going to escape extinction, he argues, they may need parasites to survive.

“Some of these parasites may turn out to be quite good for their hosts,” Dr. Spencer said.

We tend to think of species that live inside hosts as disease-causing enemies. That may be true — but only up to a point. Medical studies have shown that humans have evolved an intimate, perhaps even necessary, relationship with many of these lodgers.