Is there a cure for measles?

No. The vast majority of people who contract measles haven’t been vaccinated, and giving them the measles vaccine within 72 hours of being exposed to the virus might help — at least by reducing the severity and duration of the symptoms. The Mayo Clinic says that pregnant women, babies and people with weak immune systems can receive an injection of antibodies called immune serum globulin within six days of being exposed to measles, which might prevent or lessen the symptoms.

How safe and effective is the measles vaccine?

Extremely safe and effective. The measles-mumps-rubella (M.M.R.) vaccine causes no side effects in most children. Small numbers may get a mild fever, rash, soreness or swelling, the C.D.C. says. Adults or teenagers may feel temporary soreness or stiffness at the injection site. Rarely, the vaccine might cause a high fever that could lead to a seizure, according to the C.D.C. Contrary to misinformation that some anti-vaccine activists continue to repeat, the vaccine does not cause autism.

Children should receive two doses of the vaccine: the first when they are 12 to 15 months old; the second when they are between 4 and 6 years old. If infants who are between 6 and 11 months old are about to travel from the United States to another country, the C.D.C. recommends they receive one dose of the vaccine beforehand.

One dose of the vaccine is about 93 percent effective; two doses boost that number to 97 percent, the C.D.C. says.

People who don’t get the vaccine are at very high risk for contracting measles. “Almost everyone who has not had the MMR shot will get measles if they are exposed to the measles virus,” the C.D.C. says.

If I’m not vaccinated, is it too late to get the shot?

It’s not too late. In fact, if measles is occurring in your community, it’s a good idea to get vaccinated unless you are sure you have previously received two shots of the M.M.R. vaccine; or you’ve had all three of the diseases the vaccine protects against (which gives you lifelong immunity); or you were born before 1957. (The vaccine was made available in 1963 and in the decade before that, virtually every child got measles by age 15, so the C.D.C. considers people born before 1957 likely to have had measles as children.)

But people who received the vaccine in the 1960s should consider getting immunized again. One of two vaccines available from 1963-67 was ineffective, the C.D.C. says. The effective vaccine during those years was the “live” vaccine; the ineffective one was the inactivated or “killed” vaccine. If you’re not sure, consult with your doctor.

If most people are getting vaccinated, why does it matter if I don’t vaccinate my child?

You are probably thinking of the concept of herd immunity, which means that if a large number of people are protected from a disease by a vaccine, the disease will be less likely to circulate, diminishing the risk for people who are unvaccinated. The threshold for herd immunity varies by disease — for a highly contagious disease, a very high percentage of people need to be vaccinated to meet that threshold.

Because measles is so contagious, between 93 percent and 95 percent of people in a community need to be vaccinated to achieve herd immunity. Remember, some people can’t be vaccinated for medical reasons: infants, pregnant women and people who are immune compromised.

During the Disneyland outbreak in 2015, a 9-month-old child whose parents were planning to immunize contracted measles from an older child who hadn’t been vaccinated, said Dr. Annabelle De St. Maurice, an expert on infectious diseases at U.C.L.A. So vaccinating your child not only protects your child, it helps protect others in your community.

Wait, didn’t we eliminate measles?

In 2000, measles was declared eliminated from the United States because the country had gone for more than 12 months without any “continuous disease transmission” within its borders. “Eliminated” doesn’t mean the disease was completely eradicated; it means the United States no longer had any places where the disease was endemic or homegrown.

There have been a small number of measles cases in the United States since then, ranging from 37 in 2004 to 667 in 2014, largely among people who were not vaccinated.

Most of the American cases since 2000 have been the result of people traveling to or from countries where measles is endemic because there is little vaccination.

How many people haven’t been vaccinated?

A small number in scattered pockets. Measles immunization in the United States is stable and high — more than 90 percent — according to C.D.C. tracking.

Who are the people not getting vaccinated?

One way to measure is by looking at the annual assessment of kindergarten vaccinations. It shows only Colorado, the District of Columbia, Idaho and Kansas dipping below the national 90 percent vaccination rate, though in certain communities the rate can be lower.

Generally, those who do not immunize their children are demographically more white and more educated, Dr. De St. Maurice said. “In part due to the success of vaccines, people are not as familiar with these diseases, so they question their effectiveness,” she said. Conversely, when the disease reappears, as it did in Washington, demand for vaccines rises.

Don’t states have laws requiring parents to vaccinate their children?

Every state has these laws. Three states allow only medical exemptions: Mississippi, West Virginia and, more recently, California, following the Disneyland outbreak. The rest grant exemptions for personal, philosophical or religious beliefs as well.

Are the states with lax laws the ones with the most measles cases?

There have not been enough cases to warrant a major survey, but Dr. Saad Omer of the Emory Vaccine Center in Atlanta warned that with a rising number of cases, that could change.

Dr. Omer, who studies immunization coverage and disease incidence, said research had shown that states that allow more exemptions have increased pertussis (whooping cough) outbreaks.

Studies have also shown that people who refuse vaccines are disproportionately represented in the early stage of outbreaks. “They’re providing the tinder that can start the fires of the epidemics,” Dr. Omer said.

How does the U.S. compare with other countries?

Measles cases have been increasing around the world, too. Worldwide, there was an 80 percent drop in measles deaths from 2000 through 2017. But reported cases of measles have increased 30 percent since 2016, according to the World Health Organization.

Ninety-four percent of children in the United States get the recommended two-dose vaccine. According to the World Health Organization, more than a dozen countries reached the 99 percent mark, including Cuba, China, Morocco and Uzbekistan. Canada, Britain and Switzerland are a few of the Western countries that are below 90 percent.

In Europe overall, only about 90 percent of children receive the recommended two-dose vaccine. Worldwide, about 85 percent of children receive the first dose, but the number drops to 67 percent for a second dose, data shows.

Worldwide cases of measles have increased 30 percent since 2016, the World Health Organization said last year. Credit: Yahya Arhab/EPA, via Shutterstock