Measles Cases Are Rising Across The U.S. Here's What You Should Know.

The number of measles cases in the U.S. has reached the second-highest level in 25 years ― and it’s only April.

With the disease spreading to new states and the number of cases only expected to rise, here’s what you need to know about the infection, current outbreaks and how to protect yourself.

What is measles?

Measles is “one of the most contagious of all known infections,” according to Johns Hopkins infectious disease experts Dr. Aaron Milstone and Dr. Lisa Maragakis. The disease is caused by a virus that spreads through the air.

“Nine out of 10 unimmunized children who are in contact with an infected person will contract the virus,” according to the doctors. “The virus can linger in the air for about two hours after a person with measles has left the room.”

Measles can cause fever, runny nose and small red bumps over the entire body. In the worst cases, the disease can weaken the immune system, leading to complications like pneumonia and encephalitis, and even possibly death.

There is no prescription medication for measles. But two doses of the MMR vaccine, which protects against measles, mumps and rubella, are roughly 97% effective at preventing measles, according to the U.S. Centers for Disease Control and Prevention.

Where have measles cases been reported?

There have been 555 reported measles cases so far in 2019, according to figures released on April 11 by the CDC.

Arizona, California, Colorado, Connecticut, Florida, Georgia, Illinois, Indiana, Kentucky, Maryland, Massachusetts, Michigan, Missouri, Nevada, New Hampshire, New Jersey, New York, Oregon, Texas and Washington state have reported measles cases.

Measles outbreaks, defined as three or more cases in a given area, are currently ongoing in New York’s Rockland County, New York City, Washington’s Clark County, New Jersey’s Ocean County, Michigan’s Oakland County and several counties in California.

What’s causing the outbreaks?

The majority of recent outbreaks have been linked to travelers bringing measles back from countries with larger ongoing measles outbreaks, including Israel, Ukraine and the Philippines. Measles cases in recent years have also come from common destinations like England, France, Germany and India.

The Democratic Republic of Congo, Ethiopia, Georgia, Kazakhstan, Kyrgyzstan, Madagascar, Myanmar, the Philippines, Sudan, Thailand and Ukraine are currently experiencing widespread outbreaks.

Madagascar’s outbreak, the largest in the country’s history, has killed more than 1,200 people, most of them children, The Associated Press recently reported.

Should I get a booster vaccine?

The majority of people who become sick with measles are unvaccinated. A person who has received two doses of the MMR vaccine is considered immunized and does not ever need to get a booster shot, according to the CDC. However, the agency recommends that those who aren’t sure about their immunity or who can’t locate immunization records should consider getting vaccinated again at no risk.

How does the anti-vaccine movement play into this?

It has become increasingly common for people in the U.S. to opt out of measles immunization for themselves and their children due to religious beliefs and personal preference. Much of the anti-vaccination movement has been fueled by unfounded fears over side effects, including the conspiracy theory that the MMR vaccine has been linked to autism. That belief was largely engendered by a debunked 1998 study based on just 12 patients and conducted by a doctor who was found to have falsified data. Multiple long-term studies have shown no link between the measles vaccine and autism.

There are also portions of the population, however, who are unable to receive the vaccine for medical reasons. The vaccine is usually not given to infants less than 1 year old, in part because it’s not considered effective at that young an age. People with immune system deficiencies are also generally not given vaccines, making them more susceptible to infection if they are exposed to the disease.

That’s why doctors talk about the importance of getting vaccinated not just to protect oneself but also to boost “herd immunity,” defined by the American Academy of Pediatrics as the overall immunity individuals benefit from “as a result of living in a community where a critical number of people are vaccinated.”

What states have introduced legislation to combat the spread of the disease?

Seventeen states allow both religious and philosophical immunization exemptions in schools, according to the National Conference of State Legislatures. Many other states allow religious but not philosophical exemptions. Just three states ― California, Mississippi and West Virginia ― offer no nonmedical exemptions.

Lawmakers in Maine, Oregon and Washington are currently considering legislation to end nonmedical exemptions to immunization requirements.

The New York City health department recently ordered mandatory measles vaccinations in parts of Brooklyn to help combat a growing outbreak of the disease there.

Related Coverage

Measles Cases This Year Surpass 2018's Full-Year Total

Measles Cases Hit Second-Highest Level In U.S. In 25 years

Israeli Flight Attendant In Coma After Contracting Measles: Report

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This article originally appeared on HuffPost.