Measles is back.

The United States declared measles eradicated in 2000 as the number of people infected each year fell to zero.

But this year, 555 people have already come down with measles nationwide as of April 11. That’s 1½ times as many measles cases as there were all last year in the U.S.

California is one of 20 states where people have fallen sick with measles this year. As of Thursday, 23 people have been diagnosed with the disease, mostly in the northern half of the state.

As numbers continue to climb, we decided to answer some questions that Californians may have about measles, especially if they’re worried about catching it.

Can adults get measles?

Yes. Most of the people who have fallen sick in California this year were adults. Kids are particularly susceptible though, because they don’t receive their first measles vaccine until age 1.

In general, children younger than 5 and adults older than 20 are more likely to suffer from measles complications, such as pneumonia and ear infections that can lead to permanent hearing loss.

Measles is one of the most contagious viruses in the world and is spread through coughing and sneezing. Even after a sick person leaves a room, the virus can exist for up to two hours, waiting to infect an unsuspecting victim.

I got the vaccine when I was a kid. Could I get measles?

Probably not. The measles vaccine is one of the most effective immunizations, protecting 97% of people who come into contact with measles from getting sick. Almost everyone in the U.S. who has caught measles this year had not been vaccinated.

Is there a booster shot? Or anything else I can do to protect myself?

The vaccine keeps working for years after you get it, so no booster shot is recommended, according to the U.S. Centers for Disease Control and Prevention. If you got the recommended two doses of the MMR vaccine as a kid, you should be safe.

There is one small exception, however. People who were vaccinated against measles between 1963 and 1967 may have gotten a type of vaccine that did not work. Those people should be re-vaccinated, according to the CDC.

If you are unsure what type of vaccine you got or whether you have been vaccinated against measles at all, a doctor can check your immunity levels.

Why are these outbreaks happening?

American travelers pick up measles from other countries where the disease is spreading widely and then bring it back to the U.S. Californians caught measles this year during trips to the Philippines, India, Ukraine and Thailand, according to the state health department.

Once measles is introduced into a community, it starts to spread if people there aren’t immune to measles.

A measles outbreak this year in New York City, where 329 cases have been confirmed, began when an unvaccinated child returned from a trip to Israel, where there is a major measles outbreak. The illness then began spreading among unvaccinated members of the Orthodox Jewish community.

Measles outbreaks have been increasing in the U.S. and abroad as vaccine coverage falls. The World Health Organization named vaccine hesitancy one of the top global health threats for the first time in 2019.

Which areas are most at risk? Is my neighborhood?

People living in areas where measles is already spreading are most at risk. A list of places in the U.S. with outbreaks is available at cdc.gov/measles.

The counties in California where more than one person has been diagnosed with measles this year are Butte, Shasta, Placer, San Mateo and Santa Clara.

Communities with low vaccination rates are most susceptible to the spread of measles. Schools keep track of vaccination rates and can be a good indicator of how many people in your neighborhood are vaccinated.

You can look up how many kindergartners and seventh-graders had their shots at your kid’s schools at shotsforschools.org.

The California counties with the lowest vaccination rates among kindergartners in the 2017-18 school year were largely concentrated in the northern half of the state and include Mendocino, Sutter and Humboldt. The counties in the southern half of California with the lowest vaccination rates are Kern, San Luis Obispo and Imperial.

Is it too late for me to be vaccinated?

No. The CDC recommends that adults who aren’t immune to measles get at least the first vaccine, which grants 93% immunity.

Anyone traveling internationally or working in healthcare should get both doses, the agency says. The second vaccine, recommended 28 days after the first, will bring immunity up to 97%.

I might have been exposed to measles. What do I do now?

If you don’t have any symptoms, nothing! You’re not sick.

The signs of measles are a lot like cold symptoms, including fever and cough. They usually start about two weeks after exposure. But measles patients also get a characteristic red rash that typically begins on the face and spreads downward toward the feet.

If you have a rash, don’t run to the emergency room or urgent care. Call your doctor’s office first so they can make special arrangements for your visit so you don’t get everyone else in the office sick too.