Infections and deaths from C. difficile — the name means “difficult” — have increased sharply since the 1990s, in part because of the spread of a more virulent strain. It is estimated that several hundred thousand Americans are infected each year. Up to 1 percent of patients must have their colons removed and about 5 percent die.

While most of those infected are elderly people in hospitals or nursing homes, younger adults and children can also be infected, and there are cases that arise outside the hospital.

Problems usually start when people are treated with antibiotics for some other infection. That can kill off many of the harmless bacteria in the intestines, allowing C. difficile, which is resistant to most antibiotics, to take over.

Two drugs are now used to treat C. difficile, one of which — metronidazole, a generic antibiotic also sold by Pfizer as Flagyl — was never actually approved for this use. The other is Vancocin, an oral form of the antibiotic vancomycin, which is sold by ViroPharma and was approved in 1986.

While the drugs usually clear the diarrhea, it can come back, often more than once.

“When patients get better and are discharged and have another recurrence, it sets them back to Square 1,” said Lynne V. McFarland, an expert at the Puget Sound veterans affairs hospital in Seattle.