Measles is an extremely contagious respiratory infection caused by a virus. Once common in the United States, it was eliminated here in 2000 but is resurgent today, mostly because of the growing number of people who are refusing to vaccinate their children or delaying those vaccinations, experts say.

Here is the information you need to understand the burgeoning debate about measles.

Who gets the measles? What are the symptoms?

Mostly children, but unvaccinated adults who never have had the disease are at risk too. It starts with a fever, runny nose, sore throat and red eyes and is followed by the well-known rash of red dots that can cover your entire body, according to the Centers for Disease Control and Prevention.

But I've heard measles can be more dangerous than that.

Yes. Fatalities are rare in the United States, but one or two of every 1,000 children who contract measles will die from the disease, according to Anne Schuchat,  director of the CDC’s National Center for Immunization and Respiratory Diseases. More common complications include pneumonia, encephalitis (swelling of the brain) and deafness. Pregnant women who get the measles may give birth prematurely or deliver low-birth-weight babies.

How is it spread?

Mostly by coughing and sneezing. Also, measles can spread if you touch a surface where the virus is present and then touch your nose or mouth. The virus can live on a surface or hang in the air for as long as two hours after an infected person has coughed or sneezed. In fact, measles is so contagious that 90 percent of people who are not immune can get it from being near a single person with the disease, according to the CDC. People are contagious from four days before the rash appears until four days after it shows up.

When did the vaccine become available?

The vaccine, which uses a live virus to stimulate production of antibodies to the measles virus, became available in 1963. Widespread use began in the mid- to late 1960s. Before then, the United States averaged hundreds of thousands of measles cases every year. There were periodic outbreaks, mostly among unvaccinated children, until 2000, when endemic measles was declared eliminated in the United States.

If I've had the measles, am I immune? What if I'm not sure?

Yes, you only get measles once. If you're older than 52, chances are good you've had the measles. If you're not sure, your doctor can perform a simple blood test to determine that. If you're younger, she may just skip the blood test and give you the measles booster injection because there is no harm in doing that, according to Mark Sawyer, a pediatric infectious disease specialist at the University of California San Diego School of Medicine.

How effective is the measles vaccine?

The first dose, given at 12 months, is about 95 percent effective. The second dose, given at age 4 to 6 years, improves those odds to 98 or 99 percent. So it's possible to get the measles even if you've had the two recommended doses. But it's extremely unlikely.

If so many people are vaccinated, why is measles spreading in the United States?

Last year, there were 644 cases of measles in the United States, the most since 1994. A month into 2015, there already have been 102 cases in 14 states. The CDC says last year's outbreak — and, most likely, this year's as well — was caused by unvaccinated travelers becoming infected abroad and bringing the virus into the United States. In parts of southern and northern California, where some communities have high rates of people who have not been immunized, the disease is more easily spread. Last year, for example it tore through an Amish community in Ohio where many people had not been inoculated.

Children less than a year old typically are not vaccinated, because their immune systems are not ready. And some people, children and adults alike, cannot be immunized for medical reasons. That makes both groups vulnerable to the virus.

All 50 states require vaccination for some or all of the following diseases before children can enter school: mumps, measles rubella, diphtheria, pertussis, tetanus and polio, according to the CDC.

Why would anyone refuse to be vaccinated or to inoculate his children?

In 1998, a study published in the prestigious medical journal the Lancet suggested that the measles, mumps and rubella vaccine could cause autism. The research by Andrew Wakefield was later discredited and the study was retracted by the journal in 2010. But the paper set off a movement among people who continue to believe that there could be a link between the vaccine and autism. It has been fanned by the support of some celebrities.

Vaccination is also opposed by some people on religious grounds, and some states allow parents to opt out of vaccinating their children based on "personal beliefs." California, epicenter of the current outbreak, is one of them.

With the latest measles outbreak spreading fast, you may be surprised by the state with the best child vaccination rate in the country. (Gillian Brockell/The Washington Post)

Do children really get 10 vaccinations at once, as Sen. Rand Paul (R-Ky.) said Monday?

Close, but not really. Under the standard recommended vaccine schedule, a child could receive five injections and an oral vaccine at two months. Some of those injections cover more than one illness — the DPT injection, for example, protects against diphtheria, tetanus and pertussis (whooping cough). At 12 months, the same child might receive six injections that protect against eight diseases, if the physician didn't use a combination vaccine, Sawyer said. But most do.

However, many physicians will spread those multiple injections over a few weeks if it makes parents uncomfortable to administer so many during a single visit, Sawyer noted. Doctors are free to tinker with the schedule a bit, he said.

But that becomes a problem when some parents decide to delay vaccinations by months and even years, he said. In that case, children are going too long without critically needed protections against very serious illnesses, he said. Right now, with a measles outbreak in Southern California, Sawyer said he would not recommend even a short delay in a measles immunization.

See the vaccination schedule recommended by the CDC and the American Academy of Pediatrics here.

Isn't measles a lot worse in some other countries? Why?

Yes. Twenty million people get measles annually and in 2013, about 145,700 of them, mostly children younger than 5, died.  Consider the 2014 totals in the Philippines (53,357 confirmed cases) and China (50,960 confirmed cases), according to the World Health Organization.

In the Philippines, Typhoon Haiyan in 2013 disrupted vaccination programs and accelerated the measles outbreak there, according to Steve Cochi, senior adviser in the Global Immunization division of the CDC. In China, the disease has spread as the country failed to immunize its huge population of migrant workers, he said.

Still, conditions have vastly improved abroad as a result of a concerted vaccination campaign. The number of measles deaths worldwide dropped by 75 percent from an estimated 544,200 in 2000 to 145,700 in 2013, according to the WHO.

If I'm traveling to one of these places, what should I do? And what if I'm bringing young children?

Adults and teenagers who aren't immunized should get both doses of the vaccine, at least 28 days apart, according to the CDC. There's also no harm in any adult getting the booster. Children who are at least 12 months old should also get both doses, again separating the injections by 28 days.

Infants who are at least six months old can get one dose of the vaccine to help protect them, even though the recommended vaccine schedule doesn't call for them to be immunized until 12 months. However, those children will subsequently need two more doses to be fully protected, Sawyer said.

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