Measles: Dr. Richard Besser Responds to Your Questions

— -- Dr. Richard Besser, ABC News chief health and medical editor and a practicing pediatrician, has been getting a lot of questions on social media about the measles. He answers some here. You can ask him further questions on Twitter or Facebook.

Before the MMR vaccination program started in 1963, the Centers for Disease Control and Prevention estimates that up to 4 million people got measles each year in the United States. Between 400 to 500 people died from the disease each year, according to the agency.

Up to 40 percent of parents now delay the vaccine schedule recommended by the CDC, according to journal Public Health Reports. But a recent Pediatrics journal study found the delay doubles the risk of fever-induced seizures. Children who wait to complete their vaccines remain at increased risk for measles for a longer period of time, Besser pointed out.

There's no harm in getting a second shot if you aren't sure whether you were vaccinated as a child, Besser said. Or, ask your doctor about a blood test that determines whether you were given the vaccine.

Although the MMR vaccine is one of the most effective vaccines we have, one in 100 people who were vaccinated will still catch the virus if they are exposed to it, Besser said. About 90 percent of people who have not been vaccinated will come down with measles after exposure.

At least 90 to 95 percent of the population must be vaccinated to provide "herd immunity" everyone, studies show. Measles vaccination rates for preschoolers are below 90 percent in 17 states, a new report from the public health group Trust for America's Health found.