CHICAGO (Reuters)—Many patients infected by the deadly superbug Clostridium difficile, long thought to be contracted mainly during hospital stays, are already exposed when they are admitted to the hospital, U.S. infectious disease experts said on Tuesday.

Rates of C. difficile, the most common hospital-based infection in the United States, continue to climb. The infection can cause severe diarrhea, inflammation and bleeding in the colon, and death.

A new study from the U.S. Centers for Disease Control and Prevention found that half of the cases of C. difficile reported by hospitals were present at the time a patient was admitted or within the first three days of admission, suggesting they were already infected when they entered the hospital.

C. difficile is linked to more than 14,000 U.S. deaths each year, according to the CDC. The infections just in hospitals add an extra $1 billion a year in health system costs.

While C. difficile has long been thought to be a hospital problem, the new CDC report suggests patients can be exposed to C. difficile in many health care settings.

In a large number of cases, patients were already exposed to the infection in a nursing homes or outpatient clinics and then admitted to hospital.

"A hospital is sort of at the mercy of the amount of C. difficile that is coming in their front door," said Dr. L. Clifford McDonald, a medical epidemiologist at the CDC whose study appears in the agency's Vital Signs report.

Hospital stays involving C. difficile infections as the primary diagnosis more than tripled between 2000 and 2009, fueled in part by more deadly strains of the drug-resistant bacteria.

"C. difficile harms patients just about everywhere medical care is given," CDC Director Dr. Thomas Frieden said in a statement. "Illness and death linked to this deadly disease do not have to happen."

Dr. Frieden said infection control measures and smart use of antibiotics are needed in all of these settings to curb the spread of C. difficile.

C. difficile spreads from person to person on contaminated equipment and on the hands of health care workers and visitors.

It is especially stubborn in hospitals because of the widespread use of antibiotics, which kill protective bacteria in the gut for months, allowing invaders such as C. difficile to flourish.

"Nearly 50% of antibiotics are inappropriately prescribed, killing off the natural protective bacteria in our gut," said Dr. Jan Patterson, president of the Society for Health care Epidemiology of America.

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He said historically high rates of C. difficile illustrate the need to control the use of antibiotics.

"While we've seen other health care-associated infections decline in recent years, this one has not," Dr. McDonald said.

Dr. McDonald and colleagues at the CDC studied the problem from several different angles.

Using data gathered from some 700 hospitals reporting to the CDC's National Health care Safety Network, they found that 52% of all the C. difficile infections were present when patients arrived at the emergency department or showed up in the first three days after they arrived.

Data from CDC's Emerging Infections Program, which collects population-based data from eight geographic areas, showed that less than 25% of all C. difficile infections started in hospitals, but almost all—94 percent—started in some form of health care setting.

And a study of 71 hospitals in three states—Illinois, Massachusetts and New York—showed that following CDC guidelines for containing the infection, by wearing gloves and protective clothing and curbing the use of antibiotics, cut hospital infection rates by 20%.

The CDC recommends six basic steps for hospitals and other health facilities to prevent the spread of C. difficile:

• Prescribe and use antibiotics sparingly.

• Test patients for C. difficile when they have diarrhea if they are taking antibiotics or have been treated with antibiotics in the past several months.

• Isolate C. difficile patients immediately.

• Wear gloves and gowns even during short patient visits because handwashing and hand sanitizer may not kill the bacteria.

• Clean patient rooms with bleach or another spore-killing product.

• If transferring patients, inform the new facility of the infection.