A quarter of the infections start in the hospital, and the rest occur in nursing home patients or people recently treated in doctors’ offices or clinics. Patients often carry the germs from one institution to another.

Simple hygiene measures are highly effective, like cleaning surfaces with bleach and wearing gowns and gloves when treating infected people to avoid spreading germs to other patients. One of the disturbing and more disgusting facts about C. difficile is that it is very hard to remove from bare hands: neither soap and water nor alcohol-based hand sanitizers work very well. For health workers, it is much better to wear gloves, to avoid contaminating their hands in the first place, Dr. McDonald said.

He also said that a recent study of hospitals in the United States that set up programs to fight C. difficile found that they were able to lower infection rates by 20 percent in two years. Similar efforts in Britain have cut infection rates by half.

It is also important to use antibiotics only when they are really needed, because people taking them have 7 to 10 times the usual risk of contracting C. difficile while using the drugs and for a month after, and triple the risk for the next two months, according to the disease centers.

For those with serious illnesses that require antibiotics, the risk is unavoidable. But half the antibiotics prescribed in the United States are unnecessary, experts say, so people are being put at risk for no reason.

But why have the bacteria become more virulent? A likely reason, Dr. McDonald said, is that virulence can sometimes be an asset when it comes to evolution. Nice germs finish last, but nasty ones that cause a lot of diarrhea will spread around more, infect more people and beat out the competition — a trait that evolution will tend to favor.

“The strain that is more successful is selected for,” he said.

The finding on norovirus came as a surprise, said Dr. Hall, the C.D.C. epidemiologist, whose report is the first to find that the virus has become the second leading cause of death from gastroenteritis.