A health worker prepares a vaccine against measles, July 22, 2011. (Credit: Bernardo Montoya/Reuters)

Measles - a disease eliminated from the U.S. more than a decade ago - is making a comeback.

At least 21 people in Orange County, Calif., and 20 people in New York City have contracted the virus, which causes flu-like symptoms, a miserable rash and, in rare cases, death.

NYC Measles Outbreak Spreads to Lower East Side

Most (but not all) U.S. children receive the MMR vaccine - an immunization against measles, mumps and rubella. A single dose of the vaccine, usually administered in 1-year-old kids, is 95 percent effective in preventing measles. And a second dose virtually eliminates the risk completely.

So how did measles make its way back stateside? We imported it.

The measles is still common in other countries, accounting for 330 deaths daily, according to the World Health Organization. And because symptoms can take up to two weeks to appear, visitors can unwittingly bring it into the U.S.

If you're vaccinated, you're immune. But if you're not, there's a 90 percent chance that you'll get the virus through close contact with an infected person, according to the CDC.

Roughly 95 percent of U.S. kids are vaccinated, according to 2012 data from the CDC. But the rates vary by state.

Medical Opt-Out Rates for Vaccines Vary by State

How do you know if you're vaccinated? Every shot you get is recorded, so ask your doctor, your parents or your high school. Some states even have vaccine registries.

If you're still not sure, your doctor may be able to test your blood for antibodies, according to the CDC. Worst case: you can get the vaccine again just to be safe.

Speaking of safe, the MMR vaccine is "much safer" than getting measles, according to the CDC. The most common side effects are fever, a mild rash and swelling of glands in the cheeks and neck.