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Related media - Related media The loss of biodiversity played an especially large role in driving up disease risk, the researchers found. Many scientists have posited that biodiversity can protect against disease through a phenomenon known as the dilution effect. The theory holds that parasites and pathogens, which rely on having abundant hosts in order to survive, will evolve to favor species that are common, rather than those that are rare, Dr. Rohr said. And as biodiversity declines, rare species tend to disappear first. “That means that the species that remain are the competent ones, the ones that are really good at transmitting disease,” he said. Lyme disease is one oft-cited example. White-footed mice, which are the primary reservoir for the disease, have become more dominant o...

Related media - Associated media Dr. Clinton White, a professor of infectious diseases at the University of Texas Medical Branch in Galveston, said microscopic tapeworm eggs are sticky and easily transferred from one person to another. Once hatched, the larvae can travel in the bloodstream, he said, “and end up in all kinds of tissues.” Though it is impossible to know, he added that it is unlikely that a parasite would eat a part of the brain, as Mr. Kennedy described. Rather, Dr. White said, it survives on nutrients from the body. Unlike tapeworm larvae in the intestines, those in the brain remain relatively small, about a third of an inch. Some tapeworm larvae can live in a human brain for years without causing problems. Others can wreak havoc, often when they start to die, which c...