America’s deadliest waterborne disease is becoming more pronounced. The number of reported cases of legionellosis, an umbrella term for two illnesses caused by Legionella bacteria, climbed by 33 percent last year, to 9,933 cases, according to data released this week by the Centers for Disease Control and Prevention. The number of reported cases in 2017 was 7,458. The milder of the two ailments caused by the bacteria is Pontiac fever, which is akin to the flu. The vast majority of the cases, however, are Legionnaires’ disease, a severe respiratory illness that resembles pneumonia and kills about one in 11 people it infects. The illnesses, which are not contagious, are spread by inhaling mist contaminated with the bacteria. Rooftop cooling towers, hot tubs, showers, and ornamental fountains are among the sources of water vapor linked to disease transmission.