The measles vaccine has been available since the 1950s. It has prevented millions of cases to the point where the disease was very close to being completely eradicated. But, misinformation about the vaccine has led to an increase of cases in recent years.
"We've had 11 people hospitalized, three deaths, more than 800 cases throughout the U.S., a clear difference from years prior," said Dr. Max Brito, an infectious diseases professor at University of Illinois Chicago.
The highly contagious disease is very serious: One in five people who get it are hospitalized.
Symptoms include high fever, cough, runny nose, fatigue and rash.
"The classic rash is red dots. They start at the head, and then they go down the body," Rush Medical Center internist Dr. Elizabeth Davis said.
Davis was part of a public health team that helped contain an outbreak in Chicago last year. She said people with severe cases, which happen often, can get brain swelling and pneumonia.
"The other thing that makes measles different is people can have long-term consequences; so, years later, they can get encephalitis," Davis said.
Infectious disease experts say over 90% of people who get measles are not vaccinated.
Those who are vaccinated, but contract the disease, are usually immunocompromised. The Department of Health and Human Services Secretary Robert Kennedy Jr., a vaccine skeptic, has said vaccines are not adequately tested. Infectious disease experts strongly disagree.
"I don't think we need to prove again that measles vaccine, they work. They've been in the market for 50 years," Brito said.
Brito and Davis say it has been scientifically proven over and over that the measles vaccine does not cause autism.
"We have truly extensive data on both its safety and on its efficacy, and we know that this is a safe vaccine, and we know that this is an effective vaccine," Davis said.
If measles continues to spread, doctors are confident it can be quickly stopped. Last year, several hospitals and public health officials managed to get thousands of people vaccinated, when there was an outbreak at a Chicago migrant shelter.