The U.S. Centers for Disease Control and Prevention says 155 cases of a polio-like illness are now under investigation nationwide.
While that's up from the 127 cases it was reportedly looking into last week, no new confirmed cases have been reported.
The rare but serious illness has captured headlines, as it can cause paralysis and affects mostly children.
Sixty-two of the cases under investigation have been confirmed by the CDC as acute flaccid myelitis, or AFM, in 22 states.
On Monday the Michigan Department of Health and Human Services said the CDC is investigating four possible cases in the state.
State health officials say over the past several weeks four possible cases of AFM have been reported in Ingham, Ottawa, Macomb, and Oakland counties.
Three of the four cases involve patients under the of 18, but none of the cases have been confirmed as AFM by the CDC.
Russell Faust, the director of the Oakland County Medical Division, said what makes this illness is so scary is what we don't know:
"We don't know what causes it," he said. "At least we know it's not contagious. That should be reassuring to parents. And, it's incredibly rare."
Fewer than one in a million in the U.S. each year.
"It seems that in some cases it follows a viral illness. Everybody should keep their vaccinations up to date," Faust said.
Especially the polio vaccination, and even flu shots to minimize the risk of an AFM syndrome following a viral infection.
"There are a couple of cases that seem to be following West Nile," he added.
Faust said protecting against mosquito bites is also important, along with frequent hand washing with soap and water.
AFM affects the central nervous systems' ability to send motor signals out, so symptoms may include:
-- Sudden muscle weakness in the arms or legs.
-- Difficulty moving the eyes.
-- Drooping eyelids or facial weakness.
-- Difficulty swallowing or slurred speech.
"Polio is a known viral infection of the nervous system," Faust said.
"This is not necessary and we haven't been able to find any viral ideology. No infectious agent has been identified, and they've been looking."
If potential AFM symptoms occur, parents are urged to contact a healthcare provider immediately.