A measles outbreak in Texas and New Mexico has sickened nearly 100 people. The highly contagious virus explained — and what the CDC says about getting revaccinated as an adult.

More than 90 cases of measles have been reported in Texas since late January — marking the largest outbreak in the state in close to 30 years.

The dozens of reported cases have mostly affected children, the Texas Department of State Health Services said Friday, with 16 of the patients being hospitalized. Kids ages 5 to 17 are the most affected, with 51 cases. Twenty-six measles cases were reported among young children up to 4 years old.

Neighboring New Mexico also reported nine cases as of last Thursday, according to the state’s health department. The agency said the outbreak is happening in Lea County, near Gaines County, Texas.

The Centers for Disease Control and Prevention says measles cases have also been reported in Alaska, California, Georgia, New York City, New Jersey and Rhode Island.

Here’s what else to know about the highly contagious virus:

Where have measles cases been reported?

As of Feb. 20, the CDC says a total of 93 measles cases have been reported in eight jurisdictions, including Alaska, California, Georgia, New Jersey, New York City, Rhode Island and Texas.

What is measles?

Measles, also known as rubeola, is one of the most contagious diseases in the world, whose symptoms include fever and rash. Measles is caused by the morvillivirus, which can lead to serious health complications, especially in children under 5 who aren’t vaccinated.

“Measles is a great exploiter of unvaccinated individuals,” Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine and co-director of the Texas Children’s Hospital Center for Vaccine Development, told Yahoo News. “If you have a significant population that has not been vaccinated, then measles can rip through those populations very quickly because it’s one of the most highly transmissible virus agents we know.”

How does measles spread?

Measles is spread through the air when an infected person breathes, coughs, sneezes or talks. It’s so contagious that nine out of 10 people exposed to the virus will get infected if they have not been vaccinated, according to the CDC.

Measles “has a reproductive number of between 12 and 18,” Hotez explained. “It means a single individual can, on average, infect another 12 to 18 unvaccinated individuals.”

Airborne droplets of the virus can remain in a room for two hours, even after the person with measles has left the room, according to Cleveland Clinic. Droplets of the measles virus can also land on surfaces and spread to other people that way.

Measles can also be spread by:

• Sharing food or drinks or kissing someone who has measles

• Shaking or holding hands or hugging someone with measles

• If someone touches a surface containing the virus and then touches their mouth, nose or eyes

• Pregnant women with measles can give it to their babies either during pregnancy or while nursing

What are the symptoms of measles?

Measles symptoms can appear seven to 14 days after a person has contact with the virus. Common symptoms of measles include:

About three days after these first symptoms start, tiny white spots might appear inside the mouth.

Three to five days after the initial symptoms begin, an infected person will develop a red, blotchy rash that often starts from the head and moves down the body. The rash can last about seven to 10 days, according to Cleveland Clinic.

When are people most contagious?

People infected with measles can spread the virus to other people four days before the rash appears until four days after it goes away, according to the National Institute of Health.

What are the complications from measles and who is at risk?

Common complications from the virus include ear infections and diarrhea. Some people can suffer from severe complications from measles, including pneumonia and encephalitis (brain swelling), which could lead to hospitalization and even death, the CDC says.

Measles has the potential to be serious in people regardless of age. The following groups of people are more likely to have complications from the virus:

• Children younger than age 5

• Adults older than 20 years old

• People with weak immune systems, like cancer and transplant patients, or older adults

Is there a treatment for measles?

There’s currently no treatment for measles and the virus has to run its course, which can take about 10 to 14 days.

If a person has had a known exposure to measles, there are two types of post-exposure prophylaxis for the virus that will “potentially provide protection or modify the clinical course of disease,” according to the CDC:

• A person can get a measles vaccine within 72 hours of the initial measles exposure

• Measles immunoglobulin can be administered within 6 days of exposure

Overall, health agencies say the best treatment for measles is prevention through the measles vaccine.

What to know about the measles vaccine

There are two types of vaccines that protect against measles, given in the form of a shot:

• Measles, mumps, rubella (MMR) vaccine

• Measles, mumps, rubella, varicella (MMRV) vaccine. Varicella is the chickenpox vaccine.

When two doses are given, the vaccine is 97% effective against measles. One dose is about 93% effective.

People are often vaccinated when they are infants. The CDC recommends children get their first dose at 12 to 15 months old, with the second and final dose at 4 or 5 years old.

The CDC says a certain type of inactive measles vaccine made available between 1963 and 1967 “was not effective,” which is why they advise certain adults to get a booster. “People who were vaccinated prior to 1968 with either inactivated (killed) measles vaccine or measles vaccine of unknown type should be revaccinated with at least one dose of live attenuated measles vaccine,” the federal agency says on its website.

Regardless of age, Hotez says, “It’s never too late to go ahead and vaccinate.” He cautions Americans to make sure children are vaccinated. “It was causing half a million deaths annually in the 1980s and 1970s,” Hotez says. “It was the leading killer of children globally.”