Tough-to-treat staph infections that patients can pick up while in the hospital fell by nearly 30 percent in the last decade, according to U.S. health officials.

The biggest drop, of more than 50 percent, was seen in hospital-acquired infections, while rates of the drug-resistant infections not linked to hospitals or other healthcare settings barely changed.

"The good news is… the most serious kind of infection that lands people in hospitals and kills people is going down in the U.S.," Dr. Raymund Dantes, who led the study while at the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta, said.

He and his colleagues found that overall, there were about 80,500 methicillin-resistant Staphylococcus aureus (MRSA) infections in the U.S. in 2011, compared to about 111,300 in 2005.

Staphylococcus aureus bacteria, or staph, live on the skin or in the noses of about a third of the U.S. population and are usually harmless. But when the bacteria find their way into the bloodstream - usually through cuts and scrapes - they can turn deadly.

Symptoms of a staph infection include small red bumps on the skin, which can turn into more severe sores. When the bacteria spread past the skin, they may cause life-threatening infections in bones, organs and the bloodstream.

Staph strains resistant to the antibiotic methicillin can only be treated with one other antibiotic, and the worry is they will become resistant to that one too.

MRSA infections are often picked up while patients are in the hospital being treated for something else, or in other healthcare settings, such as dialysis centers, that people with chronic illnesses visit regularly.

Outside the healthcare arena, MRSA infections can be picked up anywhere, such as gyms or team locker rooms.

Hospitals and other healthcare providers have been making a big push to cut down on transmission of MRSA inside their facilities.

To see whether patterns of MRSA infection have changed, the researchers analyzed 2011 data on infections from selected counties in nine U.S. states, and compared it to a 2005 CDC report on MRSA incidence.

Overall, the number of serious MRSA infections diagnosed while people were in the hospital fell by 54 percent between 2005 and 2011 - from about 9.7 infections per 100,000 people to about 4.5 per 100,000 people.

The incidence of serious infections diagnosed while people were home but after being in contact with a healthcare setting also decreased, by about 28 percent, during that time - from 21 infections per 100,000 people to about 15 infections per 100,000 people.

In contrast, so-called community-acquired infections not linked to healthcare fell by just 5 percent.

While the new study cannot explain why infection rates are dropping, Dantes said it's likely attributable, in part, to hospital efforts to reduce the spread of infections.

"It's also possible that there has been evolution of these strains and they're less invasive," Dr. Franklin Lowy, from the Columbia University College of Physicians and Surgeons in New York who wrote an editorial accompanying the new study in JAMA Internal Medicine, said.

"What's interesting about our study, we didn't see a big change … in the number of community-associated infections. So we haven't made as much progress getting those down," Dantes, who is now at Emory University in Atlanta, said.

"Certainly there is more research that needs to be done to understand community-associated invasive MRSA infections. So that's an area we're looking into more," he said.

In the same journal, a separate study suggested a possible link between MRSA infections and farms.

Researchers led by Joan Casey of the Johns Hopkins Bloomberg School of Public Health in Baltimore found that Pennsylvania residents who lived closest to farms using pig manure were at 38 percent increased risk of MRSA infections, compared to those who lived farthest away.

"I think there are several possible environmental explanations," Casey said, including that the antibiotic-resistant bacteria and antibiotics from the manure may get into the air or water and end up on people's skin.

The study, however, cannot prove that living near the farms actually cause the infections.

"It raises questions about this high-antibiotic contamination of manure and its transport. Perhaps that can be another community-based reservoir for these antibiotic-resistant strains that can be then disseminated," Lowy said.

"I think at this point it's premature that someone has to be unduly concerned about it," he added.

While Dantes said it's hard to tell people how to prevent MRSA infections outside of hospitals, he said that he encourages people who are in healthcare settings to watch their doctors and nurses wash their hands.

"If you don't see them wash their hands, ask them to," he said.

Dr. Ghinwa Dumyati, an infectious disease physician at the University of Rochester Medical Center who worked on the CDC report, also said people at higher risk for MRSA infections, such as those on kidney dialysis and people with diabetes, should wash their hands too.

"We're very optimistic and there are now more and more ways to prevent the transmission," she said, adding that hospitals are experimenting with new techniques to decrease the risk for infections.