Cases of Legionnaires’ disease have been increasing dramatically in the United States, with reported cases in August alone more than doubling from expected levels for that period, US health officials say.

Infectious disease experts say they cannot pinpoint a single reason for the surge, but believe a combination of factors is at play, including an aging population that is more vulnerable to illness, better diagnostics, increased awareness of the disease, and possibly environmental factors.

University of Pittsburgh infectious disease expert Dr Amesh Adalja, speaking on behalf of the Infectious Disease Society of America, believes at least some of the increase in reported cases is related to greater awareness and the fact that doctors now routinely test pneumonia patients for the disease.