LANSING, Mich. — State officials are working with health departments in Detroit and Macomb, Wayne and Oakland counties to investigate an increase in Legionnaires’ disease cases.

The Michigan Department of Health and Human Services says Friday that 73 cases of legionellosis were identified in June and July in southeastern Michigan.

An average of 30 cases were identified over the same two months from 2014-2016.

A common source of the infection has not been found.

Legionellosis is a respiratory infection caused by Legionella bacteria. The bacteria are found naturally in fresh water lakes and streams, but also can be found in man-made water systems. Transmission to people occurs when mist or vapor containing the bacteria is inhaled.

Legionnaires’ disease is an infection with symptoms that include fever, cough, and pneumonia.