By Bonnie Berkowitz and Lazaro Gamio, Updated: Feb. 6, 2015
Unchecked, measles can travel from one person to thousands of others in less than six weeks. Even in countries with widespread immunization, outbreaks can occur in pockets of people who have not been vaccinated. More than 100 cases of measles have been confirmed in a U.S. outbreak that originated in a California theme park, most in people who had not been vaccinated, according to the Centers for Disease Control and Prevention.
Where exemptions from vaccines are allowed
Allow religious exemptions
Every state but West Virginia and Mississippi allows exemptions based on religion.
Allow philosophical exemptions
Nineteen states allow exemptions on moral or philosophical grounds.
Only medical exemptions
West Virginia and Mississippi are the only states that allow only medical exemptions.
How fast does measles spread?
These charts illustrate how measles spreads in four generations, in different vaccination scenarios.
If Patient Zero lives in a place where everyone who can be vaccinated has been, he probably won't pass the disease to others. Even if he does, an outbreak would quickly die out.
Disease spreads to 0.8 people per case on average every 10 to 14 days. Less than 0.3 percent will die (0.01-0.3%)
Pockets without vaccination
If Patient Zero is in the United States, where most people have been vaccinated but there are pockets of people who haven't, he is likely to pass the disease to another person or two. Within 42 days, a handful of people have the disease, and an outbreak is underway.
Disease spreads to 1.1 to 2 people per case every 10 to 14 days. Less than 0.3 percent will die (0.01-0.3%)
Worst-case scenario: No vaccination
If Patient Zero lives in an area that has no vaccination program, within 42 days, he can trigger an outbreak that spreads the disease to more than 6,000 people. If the area is in a developing country, where malnutrition is common and healthcare is substandard, about a quarter of those people probably would die.
Disease spreads to 12 to 18 people per case every 10 to 14 days. Up to 28 percent will die.
SOURCES: Mathematical epidemiologist Gerardo Chowell of Georgia State University; "The Clinical Significance of Measles: A Review" by Robert T. Perry, et al., published in the Journal of Infectious Diseases (2004). State department of health websites.
The U.S. measles vaccine program has reduced cases from millions in the 1950s and 1960s to a low of 37 in 2004. The disease is making a bit of a comeback, however, and parents and even doctors do not always recognize the symptoms.