The measles outbreak has now infected 94 people in eight states, with 67 cases clearly linked to Disneyland, California health officials reported Wednesday.
More cases are expected. Here are five things you need to know about measles:
It's highly contagious.
Ninety percent of people exposed to measles virus will catch it if they haven't been vaccinated or if they weren't infected before. That means 90 percent of babies under a year old will catch it if they come near enough to an infected person.
It can kill.
Many people dismiss measles as a harmless childhood illness and, for most people, it is. But for every 1,000 people infected, one or two will die from complications such as pneumonia or encephalitis. Before vaccination became common in the U.S., 400 to 500 people died every year from measles.
The vaccine works.
The influenza vaccine may not be terribly effective this year, but measles vaccines work well to protect against disease. One dose of vaccine is 95 percent protective and two doses provide 99 percent protection.
Vaccines can prevent infection, too.
The vaccine takes about two weeks to build full immunity, but if someone's been exposed to measles, getting a quick vaccine can prevent infection from taking hold.
Side-effects are rare.
In 2004, the Institute of Medicine released a report saying a careful review showed that complications from all vaccines are rare. It found the combined measles-mumps-rubella or MMR vaccine can cause fevers in some people, and seizures in a few who develop fevers. But they very rarely caused lasting effects. MMR vaccines also can cause a rare form of brain inflammation in some people with severe immune-system deficiencies; which is why experts say other people should be vaccinated to protect this subgroup of vulnerable patients.