The measles virus is one of the most infectious diseases known to humans. A person with measles can cough in a room and leave, and hours later, if you’re unvaccinated, you could catch the virus from the droplets in the air the infected person left behind.

No other virus can do that.

For anyone born before 1960, there’s a good chance they suffered through a measles infection. They may have lived to tell about it, but they probably had friends who didn’t. In the US, before a vaccine was introduced in 1963, there were 4 million measles cases with 48,000 hospitalizations and 500 deaths in the US every year. Measles was also a leading killer of children globally.

The beauty of the vaccine is that most people who get the proper doses will never get sick with measles, even if they’re exposed. And by 2000, because of widespread vaccination, the virus was declared eliminated in the US: Enough people were immunized that outbreaks were uncommon, and deaths from measles were scarcely heard of.

But that’s now changing. Globally this year, there’s been a 300 percent rise in measles cases over the same period in 2018, according to the World Health Organization. Ukraine, Madagascar, India, Pakistan, Philippines, Yemen, and Brazil are among the countries most affected, according to WHO.

But the US is also seeing a record number of measles cases. Already in 2019, health officials reported the highest number of measles cases since 1994, with more than 700 people infected.

So what’s driving the uptick here? The outbreaks here have been mostly concentrated in just four states: New York, New Jersey, Washington, and California. And among those, the vast majority of cases (474) have occurred in Orthodox Jewish communities in New York City and the New York suburb of Rockland. There, a minority of community members who are vaccine skeptical have been opting out of the shots on behalf of their children, driving vaccine rates down and creating the space for the highly contagious virus to spread.

The outbreaks have prompted extraordinary measures from health officials across the country. Both New York City, and Rockland and Washington’s Clark counties declared states of emergency over measles. Two Los Angeles universities quarantined hundreds of students of staff over potential measles exposure. President Trump, who has previously spoken out against vaccines, came to their defense last week. “They have to get the shots. The vaccinations are so important,” Trump told reporters. “This is really going around now.”

We’re burdening the health system — and a generation of doctors who have never seen measles — with a disease that absolutely could have been prevented through routine vaccination. Here are eight big questions about measles, answered.

1) How did the big US measles outbreaks happen?

Measles outbreaks in the US typically start when a traveler picks up the virus in another country where measles is still common and brings it back to an unvaccinated community here.

In New York, the current outbreaks also originated with travelers who had recently visited Israel, where a massive measles epidemic is currently underway. The travelers returned to the US and spread it among unvaccinated or under-vaccinated communities in New York state.

In the Washington outbreak, “patient zero” was also visiting from outside the country, carrying a strain of the virus that’s circulating in Eastern Europe, and came into contact with unvaccinated children in Clark County. Those children then visited public places including health care facilities, schools, and churches, as well as Ikea and Dollar Tree — spreading measles to others.

What these two outbreaks have in common: They’ve both happened in communities with high rates of people who opted out of vaccines on behalf of their children, making them more susceptible to entirely preventable diseases. And in both states, the outbreaks centered on tight-knit, traditional communities (in New York, ultra-Orthodox Jewish communities and in Washington, Slavic immigrants).

These communities have become an urgent focus of health departments across the country, said Nancy Messonnier, the director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention. Before New York and Washington, it was vaccine-refusing Amish in Ohio and Somali Americans in Minnesota. When measles strikes, outbreaks in tight-knit groups tend to be “explosive” and more difficult to control.

According to CDC data, 88 percent of all recent measles cases have happened in tight-knit communities, which Messonier defines as people of a similar background who share values and beliefs and interact often.

2) What does measles feel like, and is it deadly?

Measles is a deadly infectious disease that typically strikes children. After an incubation period of 10 to 12 days, measles comes on as a fever, cough, stuffy nose, and bloodshot and watery eyes. Loss of appetite and malaise are common too. Several days after these initial symptoms, an uncomfortable spotty rash begins to spread all over the body, starting on the face and neck and moving downward. The rash usually lasts for three to five days and then fades away.

In uncomplicated cases, people who get measles start to recover as soon as the rash appears and feel back to normal in about two to three weeks.

But up to 40 percent of patients have complications from the virus. These usually occur in the very young (children under 5), in adults over 20, and in anybody else who is undernourished or otherwise immunocompromised. Children under 5 have the highest probability of death.

The most common complication from measles is pneumonia, which accounts for most measles-related deaths. Less frequently, measles can lead to blindness, croup, mouth ulcers, ear infections, or severe diarrhea. Some children develop encephalitis (swelling of the brain), which can lead to convulsions, loss of hearing, and intellectual disabilities. Again, these complications mostly arise in people whose immune systems are already weakened because of their age, preexisting diseases, or malnutrition.

According to the CDC, the horrible mathematics of measles looks like this: One in every 20 children with measles gets pneumonia; one in 1,000 will develop encephalitis (swelling of the brain); one or two in 1,000 children will die.

3) What does a measles rash look like?

A measles rash will appear as flat red spots or small raised bumps on top of reddened skin. It usually starts on the face at the hairline and moves down the neck, torso, arms, legs, and feet.

4) How do we know the measles vaccine is safe?

Measles is prevented through the combination MMR (measles, mumps, and rubella) shot. The CDC usually recommends that children get two doses: The first dose at 12 through 15 months of age, and the second dose at 4 through 6 years of age. But amid this new outbreak, they’re suggesting babies who are traveling internationally get a first dose earlier (between 6 and 11 months).

Immunity from the vaccine lasts for decades, but you should ask your health provider about booster shots if you’re an adult. (People born before 1957, when measles was widespread, are assumed to be immune since they probably suffered through an infection. Those born in the years 1957 to 1989 may not be fully immunized, since they likely got an older. And anyone who is unsure about their vaccine status can ask for the shot now or get a blood test to check their immunity.)

The vaccine is known to be extremely safe and very effective: It contains a live but weakened version of the virus, which causes your immune system to produce antibodies against the virus. Should you be exposed to actual measles, those antibodies will then fire up to protect against the disease.

Again, side effects are rare and mostly very mild. According to the CDC, for example, fevers after the MMR vaccine occur in one in six people, and mild rashes in one in 20. More severe problems are virtually nonexistent: Serious allergic reactions happen in fewer than one in a million cases. Deafness, long-term seizures, and permanent brain damage are “so rare that it is hard to tell whether they are caused by the vaccine.” So the benefits of the vaccine — the protection of children and the communities they live in — vastly outweigh the harms.

5) How easy is it to get measles?

If you’re not vaccinated, it’s extremely easy to get measles. In an un-immunized population, one person with measles can infect 12 to 18 others. That’s way higher than other scary viruses like Ebola, HIV, or SARS. (With Ebola, one case usually leads to two others. With HIV and SARS, one case usually leads to another four.)

Measles is an airborne virus, transmitted by respiratory droplets from the nose, mouth, or throat of an infected person, so usually through coughing or sneezing.

Small-particle aerosols from someone with measles can stay suspended in air for long periods of time after the person has left a room, and the virus can live on surfaces for up to two hours.

What worries health officials is that the measles virus can spread in a person four days before the onset of the telltale rash, so people with the virus start being contagious before they’d ever know they had measles. (They stop being contagious about four days after the rash appears.)

In exceptionally rare cases, even if you are vaccinated, you can still get measles. In less than 5 percent of vaccinated people, their immune systems just don’t kick in even with the shots. Researchers aren’t sure why.

Getting only one dose of the vaccine instead of the recommended two also seems to increase people’s chances of getting measles if they’re ever exposed.

6) Is there a treatment for measles?

Unfortunately, no. Doctors can help patients avoid the more severe complications (blindness, pneumonia) by making sure patients have good nutrition and enough fluids.

For eye and ear infections that can arise, doctors can prescribe antibiotics. And because measles depletes its victims’ vitamin A levels, doctors usually give patients two doses of vitamin A supplements.

7) How common is it for parents to refuse the MMR vaccine for their children?

Overall, MMR vaccine refusal is not that common in the US. Across the country, 91 percent of young children got the MMR vaccine in 2016, according to the CDC’s latest data. That’s nearly enough for what’s known as “herd immunity”: In order for any vaccine to be effective, you need to have a certain percentage of people in a population immunized. This means diseases can’t spread through populations very easily, and it protects even those who aren’t or can’t be vaccinated, like newborns and people with vaccine allergies. And for measles, 90 to 95 percent of a population needs to get the shot.

But hidden in the statistics about state and national averages are geographic clusters of unvaccinated people.

In these clusters, vaccine refusal rates can be very high. According to a 2018 analysis published in PLOS Medicine, dozens of counties across the country had nonmedical vaccine exemption rates that were between 5.1 and 30 percent in 2016-’17. Camas County, Idaho, led the nation with a 27 percent opt-out rate.

While all 50 states have legislation requiring vaccines for students, almost every state allows exemptions for people with religious beliefs against immunizations, and 17 states grant philosophical exemptions for those opposed to vaccines because of personal or moral beliefs. (The exceptions are Mississippi, California, and West Virginia, which have the strictest vaccine laws in the nation, allowing no philosophical or religious exemptions.)

Perhaps unsurprisingly, the parts of the country that make it easier for people to opt out of their shots tend to have higher rates of ... people opting out of vaccines. The most recent 2018 analysis of US vaccine policies found that states with both religious and philosophical exemptions were associated with a 2.3 percent decrease in measles-mumps-rubella vaccine rates and a 1.5 percent increase in both total exemptions and nonmedical exemptions.

8) Will we ever fully eradicate measles?

The measles virus could theoretically be wiped off the face of the planet, never to infect another human again. That’s because it fits the profile of diseases that can be eradicated: People, and not animals, are the only carriers of the virus (so eradication wouldn’t require killing off an entire animal species), we have an effective vaccine, and we have readily available and accurate diagnostic tests to identify the disease.

Globally, there’s some good news on this front: In 2017, about 85 percent of the world’s children received one dose of measles vaccine by their first birthday, which is an increase from 72 percent in 2000, according to the World Health Organization.

But this isn’t enough for total eradication. While most of today’s outbreaks occur in developing countries, particularly in Africa and Asia, as we’ve seen in New York and across Europe, measles can take off anywhere as long as there are enough people who did not get the vaccine.

In the US, researchers have found that recent outbreaks of measles and whooping cough were indeed closely tied to people who refuse vaccinations.

To boost vaccination rates, many countries in Europe have been cracking down on vaccine-refusing parents, experimenting with an array of sanctions. Following recent outbreaks, Washington’s state Senate passed a bill to limit vaccine exemptions, while New York City is fining people who aren’t vaccinated.

But some say these measures are too little, too late. “Measles was eliminated from the US in 2000, but it’s been allowed to return,” said Hotez, in part because of “ignorant and cowardly state legislatures, and a failure by governments to mount a pro-vaccine advocacy campaign.”

Update 2/5: This article has been updated to better reflect our language standards around discussion of intellectual disabilities. We regret the oversight.