Though it proves fatal in 5% to 30% of cases, most cases can be treated successfully with antibiotics, the disease agency says.

People contract the disease when they breathe in droplets of water contaminated with the bacteria, it says.

Hospitals are vulnerable because of their complex water systems, and because many of their patients already have illnesses that could put them at increased risk of infection.

Older people, smokers, people with impaired immune systems or chronic lung disease also tend to be at higher risk,the CDC said.