Everything you need to know about the measles outbreak

The number of measles cases in the U.S. has grown to over 460 in 2019.

The number of measles cases in the U.S. so far in 2019 is now more than 460, compared to 374 cases confirmed in all of 2018, according to the Centers for Disease Control and Prevention (CDC).

Michigan has 39 cases of measles on record so far this year, the highest number of measles in the state since 1991, according to the state's Department of Health and Human Services.

New York's Rockland County issued an unprecedented state of emergency last month, banning unvaccinated minors from public places, as 153 cases of measles have been confirmed there as of late March.

While measles is in the headlines, many questions remain about what the virus is, the dangers it brings and how it can be prevented.

ABC News chief medical correspondent Dr. Jennifer Ashton appeared on "Good Morning America" Monday to break down what you need to know about what she describes as "the single most infectious virus in the world."

What is the big deal about measles?

These stats from the CDC show how serious measles really is.

- More than 100,000 people worldwide died of measles in 2017.

- Of children who are infected with measles, 1 in 10 will develop an ear infection that could lead to deafness.

- 1 in 20 children with measles will develop pneumonia that could require support in the intensive care unit.

- 1 in 1,000 cases of measles will cause brain swelling, which could be deadly.

- 1 to 2 out of 1,000 children with measles will die of the disease.

How does measles spread?

Measles is highly contagious and spreads through coughing and sneezing, according to the CDC.

Early symptoms of measles include fever, runny nose, cough, red eyes and sore throat. Those symptoms are then followed by a rash of small, red spots that spread over the body.

Why am I hearing more about measles this year?

The CDC attributes the growth of measles cases in the U.S. in recent years to two factors:

1) "More measles cases than usual in some countries to which Americans often travel (such as England, France, Germany, India, the Philippines and Vietnam) and therefore more measles cases coming into the US," according to the CDC.

2) "More spreading of measles in U.S. communities with pockets of unvaccinated people."

How do I know if I'm protected against measles?

The CDC says that written documentation showing at least one of the following means you are protected from measles:

1) You received two doses of measles-containing vaccine, and you are a school-aged child (grades K-12) or adult who will be in a setting that poses a high risk for measles transmission.

2) You received one dose of measles-containing vaccine, and you are a preschool-aged child or adult who will not be in a high-risk setting for measles transmission.

3) A laboratory confirmed that you had measles at some point in your life.

4) A laboratory confirmed that you are immune to measles.

5) You were born before 1957 (In which case are presumed to have been naturally exposed to measles.)

Keep in mind that schools, employers and doctors are only required to maintain patients' records for a limited number of years.

What is the measles vaccine?

The MMR vaccine protects against measles, as well as mumps and rubella.

The CDC recommends children get two doses of MMR vaccine, starting with the first dose at 12 to 15 months, and the second dose at 4 to 6.

Two doses of the vaccine are about 97% effective at preventing measles, while one dose is about 93% effective, according to the CDC.

Do I need an updated vaccine if I've already been vaccinated?

In short, no.

If you received two doses of measles vaccine as children according to the U.S. vaccination schedule, you are protected for life, according to CDC guidelines.

Adults like college students, healthcare personnel and international travelers who are going to be in a setting that "poses a high risk for measles transmission" need to make sure they have had two doses of the measles vaccine separated by at least 28 days, according to the CDC.

Should I get a blood test to see if I'm protected against measles?

No, according to Ashton.

"The [blood] test was designed to see if you have been naturally exposed," she said. "That's different than vaccine protection."

A blood test would detect antibodies to the measles virus. People who have written documentation of two doses of the measles vaccine do not need to get their antibody levels checked. .

Joshua Rosenblatt, M.D., a member of the ABC News Medical Unit, contributed to this report.