Minnesota health officials are investigating three cases of measles in Hennepin County in an effort to identify the source of the infections and determine if others have been exposed.

So far investigators have confirmed measles in three toddlers in Hennepin County, according to the Minnesota Department of Health. Like most new cases in the U.S., the infected children were not vaccinated against the disease.

Local and state public health departments “are following up on other potential cases associated with the confirmed illnesses,” the department said Thursday afternoon.

Measles was declared to be officially eradicated in the U.S. in 2000, but international travelers who have been exposed in foreign countries can spark new infections here when they come in contact with someone who doesn’t have immunity.

Measles is highly contagious and spreads easily by coughs, sneezes or close contact. Symptoms include a high fever, cough, runny nose and watery eyes. It is also typified by a rash that spreads from the head down to the rest of the body. Untreated, it can lead to hospitalization and death.

Minnesota sees a small number of cases each year — in the past four years there were just two cases annually. But in 2011, a small outbreak resulted in 26 cases. All were imported or were linked to foreign travel.

The best way to prevent measles is to get vaccinated from a combination shot that also protects against mumps and rubella.