Some 8,000 to 18,000 are hospitalized annually in the United States with Legionnaires' disease, according to the CDC.

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Agency: 39 people at the Quincy Veterans Home have tested positive for Legionnaires' disease

A health official warns that "we expect to see additional cases and possibly additional deaths"

Legionnaires' affects thousands annually in the United States, including a recent outbreak in New York

CNN  — 

The death toll from a Legionnaires’ disease outbreak at an Illinois veterans home spiked Tuesday to seven, with 39 others definitively diagnosed with the ailment, state health authorities said.

Tuesday’s new figures mark a significant jump from the four dead and 25 with the disease reported a day earlier. All those who died suffered from underlying medical conditions, the Illinois Department of Public Health said.

VA under scrutiny after Legionnaires’ cases in Pittsburgh

Health authorities are still awaiting test results on other residents of Quincy Veterans Home in Quincy, Illinois, a city of about 40,000 people on the Mississippi River.

“Unfortunately, we expect to see additional cases and possibly additional deaths, because the incubation period for Legionnaires’ disease can be up to two weeks, and because patients with underlying medical conditions are at increased risk of more severe illness,” said Illinois Public Health Director Dr. Nirav Shah.

On Monday, Adams County Health Department spokesperson Shay Drummond noted that the elderly are at greater risk for contracting and possibly succumbing to Legionnaires’.

Legionnaires’ disease outbreak kills 2 at Chicago hotel

The disease is not particularly rare. The U.S. Centers for Disease Control and Prevention estimates between 8,000 and 18,000 people each year in the United States are hospitalized with Legionnaires’.

It doesn’t spread from person to person but rather through the air, with those getting it typically coming down with a fever, chills and a cough. Most recover, but 5% to 30% of those who get the disease die, according to the CDC.

It can be a serious public health situation when it affects people in tight living quarters.

That’s the case at San Quentin State Prison in California, one of the country’s most well-known correctional facilities.

Six people there are confirmed to be suffering from Legionnaires’ disease, the California Department of Corrections and Rehabilitation announced Monday. Another 73 inmates are being treated for respiratory illness at the prison’s medical unit but haven’t been diagnosed.

Earlier this summer, 12 people in the South Bronx died and more than 115 people were hospitalized after contracting Legionnaires’ disease, according to the New York City health department.

As in the Illinois veterans’ home outbreak, all the dead in New York were adults with underlying medical conditions.

Hotel’s cooling tower ID’d as source of outbreak

Laboratory tests subsequently traced the outbreak to bacteria found in a cooling tower in the Opera House Hotel that matched the strain found in patients.

CNN’s Dave Stewart and Artemis Moshtaghian contributed to this article.