Seven elderly veterans have died from Legionnaires’ disease, and dozens more have contracted the ailment at a group home in Illinois, according to state Veterans’ Affairs officials, who have yet to determine the cause of the outbreak.

By Wednesday, 45 veterans had tested positive for the disease, which can cause pneumonia, according to a spokesman for the state’s Department of Veterans Affairs. The veterans all live at the Illinois Veterans’ Home in Quincy. No further veterans have been admitted to local hospitals with symptoms in the past 48 hours, the spokesman said Wednesday afternoon.

Those who have died all had underlying health problems and an average age of 88, according to the state VA spokesman, Ryan Yantis.

Legionella bacteria, which cause Legionnaires’, often thrive in lukewarm water, according to Justin DeWitt, chief engineer with the state’s Department of Public Health. The disease spreads through inhaling bacteria-laden misted water, like from a shower or hot tub, or when misted water blows onto people in public places like near fountains or commercial cooling towers. It is not transmitted person-to-person.

Late summer can be the perfect growing season for bacteria, said Mr. DeWitt, because temperature shifts can make cold water just warm enough to harbor them and slight temperature changes can upset commercial water systems and cause a spike in bacterial growth. Legionella is always present in water systems, he said, and engineers can never really purge it from systems. “Trying to eliminate it, it’s a false hope,” he said.