Sometime during December, somebody who likely caught measles abroad made a visit to one of the theme parks of Disney in California. They sneezed while there perhaps and that is likely all that it took.
That and because there are many people in California who have decided to not be immunized against measles.
The U.S. Center for Disease Control says measles is highly contagious. If one person had it, close to 90% of those close to the person who are not immune will also be infected.
The measles that began in Disney has put Orange County, California, a hotbed of the movement of anti-immunization, at the heart of the worst outbreak of measles in California in 15 years.
Since December there has been 62 cases confirmed statewide.
More cases that also originated in California spread to four states as well as Mexico.
In all, 70 have been infected, including five employees at Disney who are now back to work. Close to one quarter of the 70 who became sick needed hospitalization.
The outbreak across California is alarming health officials in the state so much that this week they warned all children less than 12 years of age and all those who are not vaccinated or otherwise immune to avoid visiting Disney parks.
The employees at Disney resorts who were in contact with the co-workers stricken with measles were asked to remain at home unless they are able to show they were vaccinated or prove via a blood test they that are immune.
The illness spread from Disney parks to 11 different California counties and to Washington, Utah, Oregon, Colorado and Mexico, said federal health officials.
The largest concentration, a minimum of 20 cases was confirmed in Orange County, which is where Disneyland is located as well as Disney Adventure Park.
That county is the center for a movement nationwide against immunizations based on them being linked to autism in a study that has since been discredited.
California requires its children to be vaccinated against measles, mumps and rubella but parents who do not believe in vaccinations can be given an exemption by signing a waiver based on personal belief.