The number of measles cases in the U.S. keeps growing. There are at least 555 confirmed cases in 20 states — the second worst year for measles since 2000, when the disease was eliminated, according to the Centers for Disease Control and Prevention. And the serious respiratory disease is surging to “alarmingly high levels” around the world, UNICEF has warned.

Unvaccinated children are most at risk, but as the disease spreads, more adults are wondering if they are protected. Some of them may not be.

On Wednesday, Santa Clara county health officials revealed that an adult visitor, possibly a worker who came down with the virus, may have infected staff at the Google campus in Mountain View, California, NBC News reported.

Last week, Rabbi Tzali Freeman, 55, said in a Youtube video that he had recently recovered from a serious bout of measles after being exposed by a visitor to his Detroit synagogue. He had been shocked to learned that the single shot he received as a child wasn't enough to protect him.

“We had been exposed to measles believing we were fully immunized when, in actuality, we were not,” Freeman said in the video. “Before we knew it, we had a first wave of 20 people in our community with measles, almost all of them the ages of 30 and 62. While a few had a mild case, for most of us it was a brutal three weeks.”

Adults with measles are 10 times more likely to be hospitalized, because they’re more likely to get pneumonia — one of the complications of the disease. For adults who want to get the vaccine, the Food and Drug Administration told NBC News Thursday that there are no shortages.

How can you tell whether you’ve been vaccinated at all or need a booster shot?

Dr. Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia and an attending physician in the hospital’s division of infectious diseases, offers advice.

What is measles and what is the recommended immunization?

Measles — a viral respiratory illness — is more contagious than Ebola, tuberculosis or influenza, according to UNICEF. It spreads through coughing and sneezing and there is no specific treatment for the disease.

To prevent getting measles, the CDC recommends children receive two doses of the MMR (measles, mumps and rubella) vaccine, starting with the first shot at 12-15 months of age, and the second dose given at 4-6 years of age.

How can adults know whether they're protected against the measles?

There are only two ways to be protected against measles: One is to have been naturally infected with the disease, which then gives you a specific immune response, and the other is to be vaccinated, Offit said.

If you were born before 1957: You’ve most likely had the measles, even if it was a very mild “subclinical” infection that produced no symptoms, so you’re almost certainly immune.

If you were born 1967-1991: The measles vaccination program started in 1963, according to the CDC. The vaccine was made better by 1967 and the first dose was routinely recommended starting that year, while the second dose — also known as the booster shot — was routinely recommended in 1991, Offit said.

“If you received one dose, you have about a 90 percent chance of being protected. Which is to say, if you were born between 1967 and 1991, you have a 90 percent chance of being protected,” he noted.

“If you were born after 1991 and have received both doses, you have about a 97 percent chance of being protected.”

How about people born in the late 1950s and early 1960s? How do they know if they're protected?

If you know you’ve had the measles at some point during your life, you’re protected. If you don’t know and you don’t have any records of being vaccinated, you may not be immune.

You could ask your doctor for a blood test to see whether or not you have measles virus specific antibodies. This is called laboratory evidence of immunity.

But Offit doesn’t recommend the test because it’s about as expensive as the vaccine and it’s not “perfect,” he said.

People who don’t have circulating antibodies in their bloodstream can still be protected if they have memory immune cells, which can’t be measured commercially, he noted.

Consider getting a first dose of the vaccine or a booster shot, especially if you’re traveling.

“I would say probably the thing that makes the most sense is that if you were going to an area where measles is common, like Europe for example, it would probably be reasonable to get a measles-containing vaccine,” Offit said.

He recommended getting it 10-14 days before the trip.

What if it's been decades since a person has had the vaccine?

If you’ve had two doses of the measles vaccine as a child, the CDC considers you protected for life.

If you're uncertain whether you've had two doses of the vaccine, should you ask for a booster?

Yes, there’s no harm in getting a booster shot at any time.

Again, people born between 1963 and 1991 likely received only one dose of the vaccine.

“So you could argue, ‘Although I have a 90 percent chance of being protected, I can put myself in an even better position by getting a booster dose,’” Offit said.

If a person doesn't have immunity and gets exposed to the measles, would getting a vaccine right away help?

Yes, because the incubation period for measles — the time from when you’re first exposed to when you get sick — is fairly long at 10-12 days.

“If you get the measles vaccine within 48 hours of being exposed, you dramatically decrease your chances of being infected. It’s actually recommended for up to five days later, but the best data are in the first 48 hours,” Offit said.