Disneyland. Expect the unexpected in the land of magic, Mickey, Minnie, and measles. Measles? In December 2014 an outbreak of measles started in Disneyland, sickening at least 75 visitors to the park. Many children, parents, and even doctors today have never seen a case of measles and never considered it a threat.

The reason for this is an extensive vaccination program that has protected most children from measles infection for the past fifty years. In the year 2000 measles was officially declared eliminated as a disease in the United States. Prior to 1963 when the measles vaccine became available about three to four million Americans contracted the disease per year and 400 to 500 died from it. Worldwide, seven to eight million children died of measles. The World Health Organization estimates that between 2000 and 2013, measles vaccination prevented 15 million deaths around the world.

Outbreaks of measles today are rare but still occur, like the one in Disneyland. Because an estimated 20 million people still contract measles each year in Europe, Asia, Africa, and other countries, an infected traveler can inadvertently bring measles into the United States and transmit it to susceptible individuals, that is, those who have not been vaccinated.

This is exactly what is believed to have happened in Disneyland in 2014. A visitor to the park, who may have recently returned from the Philippines where a major measles outbreak had occurred, carried the measles virus unbeknownst to him or her. Because a person is contagious – that is, can infect others – before the typical measles symptoms of a rash and high fever appear, this seemingly healthy person was shedding virus at every ride and concession stand, infecting dozens of unsuspecting other park visitors. These infected individuals then returned home bearing an unexpected souvenir of their vacation that infected many others in at least half a dozen states. The years 2014 to 2015 saw the largest outbreak of measles in the United States in decades.

Measles is a highly contagious disease, spread by virus-laden droplets released when an infected person coughs or sneezes. These droplets can be inhaled by others or can land on surfaces where they remain infectious for hours. Small droplets can be carried great distances by air currents, spreading the virus far and wide. This mode of virus transmission (termed indirect contact) and the fact that an infected individual can transmit the virus for several days prior to showing symptoms of the disease means that many people can be infected even before they realize that measles is among them.

Measles can now be considered a reemerging disease in the United States, that is, a disease once considered vanquished but resurges in a population. Many suspect that the reappearance of measles as a threat to the public health is the result of increasing numbers of unvaccinated individuals. Should the United States be concerned about the reemergence of measles? Why are these outbreaks occurring? Is there a danger of measles becoming an epidemic? How can an epidemic be prevented? What do we need to know about the virus that causes measles and the resulting disease in order to protect the public?