Health officials are urging parents to make sure their children are vaccinated against measles after three cases of the disease were reported in a Florida county, among more than 100 cases throughout the U.S. this year.

Federal officials declared the contagious virus had been eliminated in the U.S. in 2000; however, infections periodically occur nationwide, as the virus is still common in many other parts of the world. Travelers can bring measles into the country, where it can spread among people who are not vaccinated.

The cases in Florida’s Gulf coast county of Pinellas are the first there in 10 years.

According to the U.S. Centers for Disease Control and Prevention, measles infections have been reported in over 100 people in 21 states and Washington, D.C., so far this year. Last year, 118 cases were reported nationwide, including an outbreak involving 65 patients in Minnesota. According to a CDC report, nearly all those cases lacked immunizations, and the incorrect perception that the measles-mumps-rubella vaccine was linked to autism had contributed to the low level of vaccinations in the affected communities.

In the last month, health officials have issued warnings about measles exposure in Minnesota, Boston and Michigan after isolated cases linked to international travel were confirmed. Health officials are encouraging parents to get the recommended vaccinations for their children.

One of the Florida cases being investigated involved an unvaccinated child who was infected in the state, according to the Florida Department of Health in Pinellas County. It was not known whether the child had an exemption to Florida’s requirement for immunizations for all children attending school, Maggie Hall, spokeswoman for the Pinellas County health department, said Thursday.

Two other measles infections are being investigated in a separate Pinellas County household. The ages of those patients were not available, Hall said. They were infected in Florida and live in the same home, and neither had been immunized.

The source of the infections has not been identified, according to the health department.

“We are continuing to investigate, but we would like families to know that their children could be exposed to diseases like measles anywhere and unless they’re protected with vaccination they are risking potentially serious health effects for their child,” Dr. Ulyee Choe, an infectious disease specialist and the county’s health director, said in a statement Monday.

In July, measles infections were confirmed in two Florida residents and one visitor who were exposed to the virus while traveling in Brazil and France. Health officials said the residents had not been vaccinated and the visitor’s vaccination status was unknown.

Measles is easily spread by air droplets when infected people breathe, cough, or sneeze. Symptoms include a high fever, cough, runny nose and red, watery eyes, followed by a blotchy rash.