Measles is a highly contagious disease caused by a virus that spreads through the air. It can cause fever, runny nose and a rash of small red bumps that spread over the entire body. It can lead to serious complications, including pneumonia and encephalitis, that can result in death, particularly among young children.

The disease is no longer endemic in the United States, but the Centers for Disease Control and Prevention has linked recent outbreaks to unvaccinated travelers bringing measles back from other countries where large measles outbreaks are occurring, including Israel, Ukraine and the Philippines.

There have been at least 387 cases of measles confirmed so far this year across 15 states. Its only April, but that is already the second most reported in a single year in the U.S. since measles was declared eliminated in 2000, according to the CDC .

Measles is easily preventable with the MMR vaccine, which protects against measles, mumps and rubella. The CDC recommends that children receive two doses of the vaccine, first at 12 to 15 months of age and then at 4 to 6 years. Two doses of the vaccine are roughly 97 percent effective at preventing measles.

Officials in New York s Rockland County, which has had 157 confirmed measles cases since October, recently declared a state of emergency and took the extreme measure of barring unvaccinated minors from public spaces.

Many states, including California, are now weighing whether to make it harder to receive exemptions for MMR vaccination. Legislation introduced in California last week would give state public health officials as opposed to local doctors authority to determine which children may skip vaccinations before enrolling in school.

A bill introduced in February in Washington, where an outbreak of 49 confirmed measles cases led Gov. Jay Inslee (D) to declare a state of emergency, would remove parents ability to opt out of immunizing their children on the basis of philosophical beliefs.