View of Marigot Bay with yachts, Castries, St. Lucia island, Lesser Antilles, Windward Islands, St. Lucia
PHOTO: Image Broker/REX/Shutterstock
View of Marigot Bay with yachts, Castries, St. Lucia island, Lesser Antilles, Windward Islands, St. Lucia
Now playing
00:34
Cruise ship quarantined in port over measles concerns
PHOTO: CNN/Getty
Now playing
02:10
'Highly misleading at best': Dale reacts to Pence's op-ed
PHOTO: Gov. Cuomo's office
Now playing
03:35
Gov. Andrew Cuomo addresses women's allegations
Commanding General District of Columbia National Guard Major General William J. Walker testifies before the Senate Homeland Security and Governmental Affairs/Rules and Administration hearing to examine the January 6, 2021 attack on the US Capitol on Capitol Hill on March 3, 2021 in Washington, DC. (Photo by Greg Nash / POOL / AFP) (Photo by GREG NASH/POOL/AFP via Getty Images)
PHOTO: GREG NASH/POOL/AFP via Getty Images
Commanding General District of Columbia National Guard Major General William J. Walker testifies before the Senate Homeland Security and Governmental Affairs/Rules and Administration hearing to examine the January 6, 2021 attack on the US Capitol on Capitol Hill on March 3, 2021 in Washington, DC. (Photo by Greg Nash / POOL / AFP) (Photo by GREG NASH/POOL/AFP via Getty Images)
Now playing
03:01
DC National Guard commander: 'Unusual' Pentagon restrictions slowed response to Capitol riot
Supporters of President Donald Trump hold up their phones with messages referring to the QAnon conspiracy theory at a campaign rally at Las Vegas Convention Center on February 21, 2020 in Las Vegas, Nevada.
PHOTO: Mario Tama/Getty Images
Supporters of President Donald Trump hold up their phones with messages referring to the QAnon conspiracy theory at a campaign rally at Las Vegas Convention Center on February 21, 2020 in Las Vegas, Nevada.
Now playing
03:00
Hear why QAnon supporters believe Trump will be president on March 4th
FBI Director Christopher Wray testifies before the Senate Judiciary Committee on the January 6th insurrection, in the Hart Senate Office Building on Capitol Hill in Washington, DC on March 2, 2021. (Photo by MANDEL NGAN / POOL / AFP) (Photo by MANDEL NGAN/POOL/AFP via Getty Images)
PHOTO: MANDEL NGAN/POOL/AFP via Getty Images
FBI Director Christopher Wray testifies before the Senate Judiciary Committee on the January 6th insurrection, in the Hart Senate Office Building on Capitol Hill in Washington, DC on March 2, 2021. (Photo by MANDEL NGAN / POOL / AFP) (Photo by MANDEL NGAN/POOL/AFP via Getty Images)
Now playing
02:55
Watch FBI director debunk conspiracy theories pushed by Trump supporters
WASHINGTON, DC - FEBRUARY 10: Neera Tanden, nominee for Director of the Office of Management and Budget (OMB), testifies at her confirmation hearing before the Senate Budget Committee on February 10, 2021 at the U.S. Capitol in Washington, DC. Tanden helped found the Center for American Progress, a policy research and advocacy organization and has held senior advisory positions in Democratic politics since the Clinton administration. (Photo by Anna Moneymaker-Pool/Getty Images)
PHOTO: Anna Moneymaker-Pool/Getty Images
WASHINGTON, DC - FEBRUARY 10: Neera Tanden, nominee for Director of the Office of Management and Budget (OMB), testifies at her confirmation hearing before the Senate Budget Committee on February 10, 2021 at the U.S. Capitol in Washington, DC. Tanden helped found the Center for American Progress, a policy research and advocacy organization and has held senior advisory positions in Democratic politics since the Clinton administration. (Photo by Anna Moneymaker-Pool/Getty Images)
Now playing
01:33
Neera Tanden releases statement on pulling her nomination
PHOTO: KCAL/KCBS
Now playing
01:41
Multiple people killed in crash after SUV and semitruck collide
nigeria kidnapped schoolgirls released Busari pkg intl ldn vpx_00000423.png
nigeria kidnapped schoolgirls released Busari pkg intl ldn vpx_00000423.png
Now playing
02:09
Tears of joy and relief as 279 Nigerian schoolgirls return home
New satellite images taken by Maxar show that North Korea sometime in the past year built a structure that may be intended to obscure entrances to an underground facility where nuclear weapons or nuclear weapons components are stored.
PHOTO: Courtesy Maxar
New satellite images taken by Maxar show that North Korea sometime in the past year built a structure that may be intended to obscure entrances to an underground facility where nuclear weapons or nuclear weapons components are stored.
Now playing
01:47
See images US intelligence claims is a secret weapons site
NEW YORK, NY - JULY 23: New York Gov. Andrew Cuomo speaks during the daily media briefing at the Office of the Governor of the State of New York on July 23, 2020 in New York City. The Governor said the state liquor authority has suspended 27 bar and restaurant alcohol licenses for violations of social distancing rules as public officials try to keep the coronavirus outbreak under control. (Photo by Jeenah Moon/Getty Images)
PHOTO: Jeenah Moon/Getty Images
NEW YORK, NY - JULY 23: New York Gov. Andrew Cuomo speaks during the daily media briefing at the Office of the Governor of the State of New York on July 23, 2020 in New York City. The Governor said the state liquor authority has suspended 27 bar and restaurant alcohol licenses for violations of social distancing rules as public officials try to keep the coronavirus outbreak under control. (Photo by Jeenah Moon/Getty Images)
Now playing
02:29
NYT: Third woman comes forward against Gov. Andrew Cuomo
PHOTO: Courtesy Penguin Random House
Now playing
01:00
These 6 Dr. Seuss books won't be published anymore
John King Magic Wall 0301
PHOTO: CNN
John King Magic Wall 0301
Now playing
03:00
US coronavirus numbers coming down, but not enough
FILE - In this Jan. 29, 2011 file photo, Saudi journalist Jamal Khashoggi speaks on his cellphone at the World Economic Forum in Davos, Switzerland. Saud Al-Mojeb, Saudi Arabia's top prosecutor, is recommending the death penalty for five suspects charged with ordering and carrying out the killing of Saudi writer Jamal Khashoggi. Al-Mojeb told a press conference in Riyadh Thursday, Nov. 15, 2018,  that Khashoggi's killers had been planning the operation since September 29, three days before he was killed inside the kingdom's consulate in Istanbul. (AP Photo/Virginia Mayo, File)
PHOTO: Virginia Mayo/AP
FILE - In this Jan. 29, 2011 file photo, Saudi journalist Jamal Khashoggi speaks on his cellphone at the World Economic Forum in Davos, Switzerland. Saud Al-Mojeb, Saudi Arabia's top prosecutor, is recommending the death penalty for five suspects charged with ordering and carrying out the killing of Saudi writer Jamal Khashoggi. Al-Mojeb told a press conference in Riyadh Thursday, Nov. 15, 2018, that Khashoggi's killers had been planning the operation since September 29, three days before he was killed inside the kingdom's consulate in Istanbul. (AP Photo/Virginia Mayo, File)
Now playing
02:52
3 names mysteriously removed from Khashoggi Intel report
Protesters take cover behind homemade shields as tear gas is fired during a demonstration against the military coup in Yangon on March 1, 2021. (Photo by STR / AFP) (Photo by STR/AFP via Getty Images)
PHOTO: STR/AFP/AFP via Getty Images
Protesters take cover behind homemade shields as tear gas is fired during a demonstration against the military coup in Yangon on March 1, 2021. (Photo by STR / AFP) (Photo by STR/AFP via Getty Images)
Now playing
02:16
Footage shows tear gas, flash bangs used on protesters in Myanmar
New York Gov. Andrew Cuomo speaks during a news conference at a COVID-19 vaccination site in the Brooklyn borough of New York, Monday, Feb. 22, 2021. (AP Photo/Seth Wenig, Pool)
PHOTO: Seth Wenig/Pool/AP
New York Gov. Andrew Cuomo speaks during a news conference at a COVID-19 vaccination site in the Brooklyn borough of New York, Monday, Feb. 22, 2021. (AP Photo/Seth Wenig, Pool)
Now playing
01:12
Gov. Andrew Cuomo responds to allegations of sexual harassment
 Psaki biden White House Khashoggi Saudi Arabia sotu bash vpx _00011629.png
PHOTO: CNN
Psaki biden White House Khashoggi Saudi Arabia sotu bash vpx _00011629.png
Now playing
03:42
Bash to Psaki: Why hasn't Saudi Arabia been held accountable for murder of Khashoggi?
(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.

How contagious is it?

“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.

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.

I know I was vaccinated. Am I protected?

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.

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.

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.

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.

Get CNN Health's weekly newsletter

Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.

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.”

CNN’s Dr. Edith Bracho-Sanchez and Sandee LaMotte contributed to this report.