California’s hospitals are getting better at preventing patient infections, but with nearly 20,000 infections reported in 2015, the threat is still significant, according to state data released Friday.

From 2014 to 2015, 56 California hospitals demonstrated “significant improvement” in preventing certain infections, including ones in the blood and those resulting from surgery, according to the report from the California Department of Public Health.

But diarrheal infections in California hospitals have been harder to contain, rising 8 percent above a national average since 2011. They now are the most common hospital infection.

Hospitals in California and nationwide have spent millions of dollars to fight hospital-acquired infections. About 1.7 million of these infections strike hospital patients nationwide each year, resulting in nearly 100,000 deaths and an estimated $20 billion in extra health costs, according to the U.S. Centers for Disease Control and Prevention.

A recent federal report found that between 2010 and 2015, about 125,000 lives and $28 billion were saved because of aggressive hospital infection control programs.

In California, bloodstream infections have dropped 10 percent below a national average since 2011, according to the CDPH data.

Infections of the blood result in thousands of deaths each year even though they are preventable, according to the CDC. Proper hand hygiene and using only sterile devices are two ways to prevent these infections, the CDC says.

The agency released a second report Friday showing that flu vaccination rates among hospital employees have risen 21 percent since 2011.

Gerard Brogan, a registered nurse and representative of the California Nurses Association, said there is more awareness of the importance of flu shots, especially for employees in hospitals and other health care settings.

But while flu shots for nurses and other frontline caregivers are recommended, “we don’t think it should be a condition of employment,” Brogan said.

In the 2015-2016 flu season, 34 of California’s 58 counties required hospital employees who had declined vaccinations to wear surgical masks.

The vaccination rate among hospital workers in the counties that enforced such a policy was 82 percent, compared to 79 percent in counties that did not, according to the new CDPH data.

The flu is the eighth-leading cause of death in the U.S. Since 2008, California hospitals have been required to offer free vaccines to their employees.