“Measles is one of the most highly contagious diseases known,” said Dr. Julia S. Sammons, a pediatric infectious disease specialist and medical director of the Department of Infection Prevention and Control at the Children’s Hospital of Philadelphia.
Once a person has measles, about 90% of close contacts who are susceptible to it will develop the disease, she added.
The measles virus spreads through coughing and sneezing. Afterward, it can linger in the air for up to two hours. If someone who is not immune to the virus breathes the air or touches an infected surface, they can become infected, according to the US Centers for Disease Control and Prevention.
How do I know if I have measles and not just a cold or the flu?
Early on, measles can be confused for other viral illnesses such as the flu. But the red blotchy rash that comes with it may help set it apart.
“Imagine that you have a bucket of rash. If you pour that bucket of rash over your head, the rash sort of cascades down,” Sammons said. “So the rash starts over the head, typically over the scalp, and then it spreads down head to toe.”
In addition, the virus often manifests as a combination of high fever — as high as 105 degrees Fahrenheit — along with cough, runny nose and pink eye, Sammons explained. Some people may develop blue-white spots on the insides of their mouths.
Anyone who’s infected can still be contagious for four days before and after a rash appears, experts say.
Another type of post-exposure prophylaxis, called immunoglobulin, is also sometimes given to folks who face an increased risk of serious illness or complications down the line, including babies under 12 months and pregnant women who aren’t verifiably immune.
Before running to the doctor, however, you might want to call ahead.
“What you don’t want to do is to go to a busy pediatric waiting room, for example, and potentially expose others,” said Sammons, urging anyone who’s concerned to speak with a care team for guidance before coming in.
I know I was vaccinated. Am I protected?
Prior to 1989, a single-dose recommendation had been in place starting in 1963.
But if you’ve had one documented dose of the effective “live” vaccine and aren’t at high risk of exposure, the agency says that’s adequate.
Those at higher risk — whom the agency advises to get two doses for good measure — might work in health care, travel internationally or be more likely to be affected by an outbreak.
I’m not sure if I was vaccinated. What should I do?
Excepting people who work in health care, most people born before 1957 don’t need the vaccine because “before vaccines were available, nearly everyone was infected with measles, mumps, and rubella viruses during childhood,” the agency says, resulting in lifelong immunity.
Before we had a vaccine, the agency says 3 million to 4 million Americans were infected yearly, including 48,000 hospitalizations and 400 to 500 deaths.
But what if you don’t have written documentation of the right vaccine? For anyone who’s unsure, the CDC says you can simply roll up your sleeve for another dose or two.
“The MMR vaccine is safe, and there is no harm in getting another dose,” the agency says.
There is also a blood test doctors can use to check immunity. Though some experts say it’s impractical or expensive for the general population, versus simply getting another shot.
Some people can’t get vaccinated at all or need to wait: for example, people with weakened immune systems and babies who are too young to respond to the vaccine.
Is it risky to travel overseas?
Most cases of measles in the United States stem from someone who isn’t protected by the vaccine traveling abroad, according to the CDC. That person can then spread it to others when they get back. The virus is commonly found in various countries in Europe, Asia and Africa — including large current outbreaks in Israel, Ukraine and the Philippines.
The “contact zone” typically includes folks sitting in the same row as whoever was infected, plus two rows ahead and behind. One exception: It doesn’t matter where a child under age 2 was sitting on the plane; they will be considered contacts no matter what.
What can I do to protect myself in public spaces?
First item on doctors’ lists: Make sure you’re vaccinated.
The CDC says you can take other steps, as well: wash hands often or using sanitizer, avoid touching your eyes and mouth, disinfect surfaces and toys with standard household products, and refrain from coming into close contact or sharing silverware with anyone who’s sick.
And if you have to cough or sneeze, use your sleeve or a tissue — but not your hands, the CDC says.
If you think you’ve been exposed to measles, the agency recommends calling your doctor.
For people in the midst of an outbreak who can’t get vaccinated, are unsure about their status or think they might be infected, public health experts recommend limiting contact that could spread the virus — including keeping unvaccinated kids home from school.
In some cases, officials may also issue a quarantine — an effective, if sometimes controversial, measure to limit the spread of an infectious disease. Every state has laws in place that allow quarantines and other public health enforcement tools, and they differ based on the jurisdiction.
“Campuses really are hotbeds of infectious diseases,” said Georgetown University’s Lawrence Gostin, who directs the O’Neill Institute for National and Global Health Law. “Young people are in close contact, being intimate, eating food together, living together in dorms.”
CNN’s Dr. Edith Bracho-Sanchez and Sandee LaMotte contributed to this report.