(CNN) A series of measles cases in the United States has led to fear and uncertainty in some communities, as well as quarantines at sea and at universities. Some cases have even hit adults who thought they were protected by the vaccine.
Even the Avengers could hardly be prepared for the threat, with moviegoers in California facing possible exposure to the virus. Same for some travelers at multiple airports across the country.
In 2019, the US has seen the highest number of confirmed cases since disease was declared eliminated in 2000. Some officials across the country have declared states of emergency.
"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.
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.
Even after being exposed to the virus, the vaccine may offer some protection or make the illness milder when given within three days. Symptoms may not appear until one or two weeks following infection.
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.
If you were vaccinated with two doses, the CDC says you have a 97% chance at being protected against measles. And if that last few percent happen to come into contact with the virus, they're less likely to spread it to others, and their illness is often milder. One dose is still about 93% effective at preventing the disease.
The current recommendation for two doses was issued in 1989 by the CDC. Doctors give the first dose of the MMR vaccine -- so called because it covers measles, mumps and rubella -- between 12 and 15 months of age. The second is given between 4 to 6 years.
Prior to 1989, a single-dose recommendation had been in place starting in 1963.
But there's a snag: From 1963 to 1967, doctors used different types of measles vaccines before discovering that one -- the "killed" version -- was ineffective. Folks who received that vaccine, or who were vaccinated during those years and aren't sure which one they got, should get vaccinated anew, the CDC says.
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.
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.
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.
And even when traveling internationally with a young child is unavoidable, the agency recommends that infants between 6 and 11 months get one dose of the vaccine before hopping on a plane -- before the usual first dose at 12 to 15 months.
Like other enclosed and crowded spaces, planes can create opportunities for transmission, though the CDC says the overall "risk of getting a contagious disease on an airplane is low." Still, the risk surged to the forefront when one recent case left an Israeli flight attendant in a coma.
When the CDC and other public health authorities are called to investigate possible transmission on aircraft, they try to figure out who may have had contact with the infected person -- including people who may have been sitting nearby. In these cases, the flight manifest comes in handy, as does listing your up-to-date contact info when you book a flight.
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.
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.
Such was the case with more than 1,000 students at two California universities last month as officials raced to contain a potential measles outbreak. Those who couldn't verify their immunization records were asked not to return to work or school. For some, this meant staying in quarantine for just a couple of hours; for others unable to get a hold on those records, the quarantine lasted much longer.
"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."