On Jan. 16, Robert Sears (Dr. Bob), a celebrity pediatrician, made a Facebook post encouraging his followers not to “let anyone tell you you should live in fear” of measles. He said if older grandparents are asked about measles they will say “so what? We all had it. It’s like chicken pox.”

Many health professionals, including the Centers for Disease Control (CDC) disagree. The California Department of Health issued an official warning that any place where large numbers of people gather is a danger to contracting measles for the unvaccinated. This includes shopping malls, airports and tourist attractions (such as Disneyland, where the current outbreak originated).

Grandma and Grandpa are correct, in that for most people measles is not life-threatening, although it does make the victim miserable. Most people have symptoms including runny nose, cough, fever and a rash all over the body. However, some people get a great deal sicker and can suffer complications such as encephalitis (a life-threatening brain condition) and pneumonia. The disease can cause pregnant women to deliver premature babies.

Before a measles vaccine was developed, as many as 500 Americans per year died from the disease. Between 3 and 4 million Americans would contract the disease, with 48,000 hospitalized and 4,000 developing encephalitis.

Measles is one of the most contagious illnesses in existence. According to the CDC, 90 percent of unvaccinated people who come in contact with the virus will be infected. It is also particularly dangerous because it can live on surfaces, or even in the air, for up to two hours.

The measles vaccine is 97 percent effective, according to health officials. The vaccine was licensed for use in the U.S. in 1963, and cases plummeted as increasing numbers of children were vaccinated. Ten years ago there were fewer than 100 cases per year, and in 2000 the disease was declared eradicated in the U.S.

“Herd immunity” can protect those who are unvaccinated. If enough people in a community are vaccinated, then those who cannot get the vaccine, including those too young or too sick, are still protected. The threshold for herd immunity for measles is 92 to 94 percent of people vaccinated. However, in some places in the U.S. the measles vaccine rates are nowhere near the levels needed for herd immunity. In Marin County, Calif., only 80 percent of students are current on vaccinations. In Nevada County, Calif., it is only 73 percent. Dozens of New York City private schools reported last year that their immunization rates were below 70 percent.

Worldwide the disease is far from eradicated. The CDC says that in 2013 more than 60 percent of children in Ethiopia, Indonesia Nigeria, Pakistan, India and the Democratic Republic of Congo were not vaccinated adequately against measles. Seventy percent of worldwide measles deaths occurred in these countries. Travelers infected abroad can bring the virus back into the U.S. and spread it, which is what California health officials think happened with the Disneyland outbreak.

As more parents file for exemptions to the requirements that their children be vaccinated there are concerns that the measles could make a major comeback in the U.S., as happened in the U.K. in 2008 and in France between 2008 and 2011. The U.S. saw 644 cases of measles in 27 states last year, the most since 1994.