A child in Somerset County who had been vaccinated contracted the virus.
State Epidemiologist Dr. Siiri Bennett said Maine CDC first became aware of this measles case earlier this week.
"The problem with something like measles is that you can actually spread the infection before you develop symptoms. They may not know that they've got measles but they can spread it," she said.
What makes this case unusual is that the infected child was vaccinated, something experts say only happens to about three out of 100 people who contract the disease.
"It's unclear to experts if the immune system really hasn't responded to the vaccine or maybe the vaccine's response has waned over time. But we do see this," Bennett said.
While only one case has been reported so far, Maine CDC officials said the infected child visited a number of places in Madison, Skowhegan and Waterville in late April and early May, including Redington-Fairview General Hospital, Madison Junior High School and Madison High School.
"I haven't had any parent concerned, actually. Our attendance is outstanding - the best it's been all year. But it's business as usual here," Madison Junior High School Principal Ryan Arnold said.
Madison school officials said they are not as concerned about a measles outbreak there because less than 1 percent of their population is unvaccinated, which they said is down from 10 percent just a few years ago.
"It's not really a concern. The kids aren't nervous -- smiles on all their faces," Arnold said.
Despite that, school officials are taking precautions, including wiping down handles and desks.
Maine CDC is asking anyone who believes they were exposed to take precautions as well by calling a doctor if they get symptoms and staying up to date on vaccinations.
"It's still the best way to prevent transmission and to reduce the number of cases," Bennett said.