Here's what to know if you're unsure about your measles immunity status

Measles is the most contagious disease known. It can rip through a population without immunity like wildfire. Just look at what's happening in Pennsylvania, says Patsy Stinchfield , executive director of the Measles Collaborative, a public health effort led by the National Foundation for Infectious Diseases.

"In January in Pennsylvania, they had 12 cases. Now we're seeing 100 plus cases per week and over 1,000 cases in Pennsylvania," she says. "If Pennsylvania can see it spread that fast, that can happen in any state that has low immunization rates."

As vaccination rates have fallen across the country, many communities have become vulnerable. Nationwide, more than 4,000 measles cases have been reported this year alone — the highest number in 35 years. Pennsylvania has reported five deaths. New York state declared a measles emergency.

Stinchfield says the recent measles surge has got many people wondering if they're at risk.

"It's a very common question for people to ask, 'am I immune? Am I OK? What do I need to do?' And that is exactly the right question everyone should be asking themselves right now," Stinchfield says.

The best defense is vaccination. Two doses of the measles, mumps and rubella vaccine is 97% effective — meaning 97% of people will have an immune response.

"The measles vaccine is safe. It's highly effective, and it confers lifelong immunity," says Dr. Adam Ratner, a pediatric infectious disease physician in New York City and author of Booster Shots , a history of measles.

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