(CNN) Residents of Flint, Michigan, already reeling because of lead contamination in their drinking water, got more disturbing news on Wednesday.

Gov. Rick Snyder announced the number of cases of Legionnaires' disease has spiked in Genesee County in the two years since Flint switched its water supply from the Great Lakes to the Flint River.

But the increase cannot be directly attributed to the switch, said Nick Lyon, director of the Michigan Department of Health and Human Services. Not all the people who got Legionnaires' were exposed to Flint water, he said.

From June 2014 to November 2015, at least 87 county residents developed Legionnaires' disease, compared to between six and 13 cases in the four preceding years, Lyon said. Ten patients died, he said.

The water looked smelled and tasted funny

Health officials are not changing recommendations about how Flint residents should deal with the water crisis. They should keep drinking bottled water and use water filters, Snyder said.

Legionnaires' disease is a respiratory bacterial infection usually spread through mist that comes from a water source. It is not transmitted person to person. Symptoms of the disease include fever, chills and a cough. The CDC estimates 8,000 to 18,000 people are hospitalized with Legionnaire's disease in the United States every year.

Flint, an economically depressed city of about 100,000 people, started getting its water from the Flint River about two years. It previously bought Lake Huron water through the city of Detroit. The state government made that change to cut costs.

Soon after the switch, the water started to look, smell and taste funny. People complained.

On January 5, Snyder declared a state of emergency for Genesee County as a result of the water crisis.

He apologized for the state's actions, but residents and officials are furious. The state denied the problem existed for months and failed to use anti-corrosive treatment on the river water, which might have reduced the iron.

'Flint residents are victims in this crisis'

"Flint needs more action and less talk from Governor Snyder," U.S. Rep. Dan Kildee said in a press release. "It is important to remember that this crisis was created by a state-appointed emergency financial manager, and it is the state's ultimate responsibility to act and make it right. Flint residents are the victims in this crisis, and they deserve a more urgent response equal to the gravity of this crisis."

Flint switched back to Detroit water in October.

"It will require more time and effort before the water is safe to drink again," the city of Flint website said . "Until that time, residents and water customers are urged to continue using water filters and bottled water."

Dr. Marc Edwards, a Virginia Tech researcher who studied Flint water and found high levels of lead, said it would be almost impossible to make a definitive link between the water source and Legionnaires' disease. To do so, he said, would require matching the strain of Legionnaires' in someone's body to the strain in the water.

"What's clear is that there's an association, which means that the increase of the Legionnaires increased pretty dramatically," he said. "And there's a strong likelihood that it's related to the water supply.

"We'll never know for sure, but we did find very high levels in the time period when they were on the Flint River water."

Edwards said this might be the first case of Legionnaires' associated to lack of corrosion control.

"This is already a tragedy," he said. "It's a crisis of conscience in government. This could potentially just add to it. But they're being very forthcoming about it at this point. There are just so many places where you wish people had done things differently."

National Guard activated

Snyder activated the state National Guard on Wednesday to assist with the water crisis.

Seven guardsmen arrived in the town Wednesday, and up to 30 are expected by Friday, Snyder said in a news release.

They will help distribute bottled water and water filters at fire stations and other public buildings, he said. That will allow Red Cross volunteers to go door-to-door to hand out water, filters and testing kits.

FEMA announced that it approved Snyder's request for federal assistance, according to agency spokesman Rafael Lemaitre. The governor had asked FEMA to step in to coordinate with other federal agencies.