More than 100 people across 21 US states and Washington, DC have reported measles cases this year, the Centres for Disease Control and Prevention (CDC) has said.

From January to 14 July this year, the US agency reported that 107 people have been diagnosed with the contagious virus in DC and the following states: Arkansas, California, Connecticut, Florida, Illinois, Indiana, Kansas, Louisiana, Maryland, Michigan, Missouri, Nevada, New Jersey, New York, North Carolina, Oklahoma, Oregon, Pennsylvania, Tennessee, Washington and Texas.

Measles had been declared “eliminated” from the US in 2000, meaning the disease was no longer continuously being transmitted within the country. But American unvaccinated travellers who get measles while visiting other countries can sometimes spread the disease to other unvaccinated people when they return, which sometimes leads to an outbreak.

The CDC has cautioned that a measles endemic, which would mean its regularly found in a particular area, could re-establish itself in the US.

“Research shows that people who refuse vaccines tend to group together in communities,” the agency states. “When measles gets into communities with pockets of unvaccinated people, outbreaks are more likely to occur. These communities make it difficult to control the spread of the disease. And these communities make us vulnerable to having the virus re-establish itself in our country.”

The CDC lists fever, runny nose, cough, red eyes and sore throat as early symptoms of measles. It warns that the disease is highly contagious and recommends vaccination. The agency recently cited to data of previous outbreaks and declared that the majority of people who got measles were unvaccinated.

A record number of people were diagnosed with measles in 2014; there were 667 cases of the disease reported across 27 US states. It was the largest outbreak of measles in the US since it was eliminated more than a decade prior. The agency attributed the cause for a majority of those cases to one large outbreak in an unvaccinated community in Ohio.

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The CDC recommends people get a Measles, Mumps, and Rubella (MMR) vaccination, which is a “weakened” live virus vaccine. The agency states the vaccine causes a "harmless infection" in the body with few, if any, symptoms before the person’s immune system fights the infection, which develops their immunity.