As Mr. Bocci learned, C. diff symptoms are to ordinary diarrhea as a nuke is to a homemade firecracker. “People who have open-heart surgery and take antibiotics — and then get C. diff — tell me they’d rather have the surgery again than C. diff,” said Dr. Dale Gerding, who directs the research lab at the Edward Hines Jr. Veterans Affairs Hospital in Chicago.

Most cases arise after a course of antibiotics (the class called fluoroquinolones is a leading culprit) wipes out the protective microbes normally found in the colon. That allows C. diff, most commonly acquired from health care providers and facilities, to flourish and produce the toxins that cause illness.

Like many diseases, it hits older people harder. They are more likely to be exposed to the spores in hospitals and nursing homes, and more likely to be prescribed antibiotics.

“In the Medicare population, the rates are many times higher than in younger patients,” Dr. Gerding said. “The recurrence rate also goes up. Mortality goes up probably 10 percent with every decade.” More than 80 percent of deaths associated with C. diff occur in those over 65, according to C.D.C. data.

If infection rates are indeed declining, Dr. Gerding said, one reason is that the NAP1 strain appears to be ebbing. But campaigns to reduce the overprescription of antibiotics have also gained traction.

Most hospitals have established antibiotic stewardship programs to track and control their use, and new federal regulations require such programs in nursing homes, too. English researchers credit reduced antibiotic use with cutting C. diff rates by 80 percent.

Patients can also play a role by not demanding unnecessary antibiotics. They are useless for many transient illnesses like respiratory infections and leave people vulnerable to pathogens.