Many parents seek a better understanding of measles – a potentially dangerous, highly-contagious viral infection that affects children and adults.

Johns Hopkins infectious disease experts Aaron Milstone, M.D., M.H.S., and Lisa Maragakis, M.D., M.P.H., provide answers to some common questions.

Who is most at risk for measles?

Unvaccinated children are at risk of getting measles, but so are other people who are not able to get the vaccine or who have compromised immune systems.

How does the spread of measles happen?

Measles is a highly contagious viral infection — one of the most contagious of all known infections. Nine out of 10 unimmunized children who are in contact with an infected person will contract the virus. The virus can linger in the air for about two hours after a person with measles has left the room. It can infect those who enter the room if they are unimmunized.

People who travel to areas with known measles cases are at risk of getting the disease. People who travel outside the country to areas where measles is common bring it into the U.S. Getting the measles vaccine is the best way to protect yourself. In the United States, the vaccine is not given to babies under 1 year of age because they are too young to be immunized.

Vaccines are also not given to those with immune system deficiencies. These people are susceptible to measles infection if they are exposed. A growing number of people in the United States opt out of measles immunization because of personal preference. The majority of people who get measles in the U.S. are unvaccinated. When one or two infected people come into contact with a population that includes unvaccinated people, the virus can spread rather quickly.

How dangerous is infection from measles?

At best, measles is a very uncomfortable illness. At worst, it can be lethal. Most commonly, the infection causes high fever, cough, conjunctivitis (red, runny eyes), runny nose and a rash that begins on the face and eventually covers the entire body. The illness lasts about a week if there are no complications.

When complications do occur, they can include ear infections, pneumonia, and encephalitis or inflammation of the brain that can lead to permanent neurologic damage and even death. On average, measles kills between one and three of every 1,000 infected children.

If exposed to measles, is there any cause for concern if vaccinated?

The measles vaccine creates lasting protection in 95 percent of children who receive one dose of it and in 99 percent of those who receive the second dose. It is rare for an immunized child to develop infection after exposure to someone with the disease.

How does measles affect babies who have only completed part of the immunization schedule?

Even a single dose of the vaccine is highly effective in preventing infection. Under usual circumstances, it is recommended that the measles vaccine be given at 12 to 15 months and again at age 4 to 6. If exposure to measles is likely, however, either because of planned travel to an area where measles occurs more frequently, or because of possible exposure during an outbreak, the second dose can be given as soon as one month after the first, leading to 99 percent protection.

Also, when exposure is more likely, infants between 6 and 12 months of age can be immunized. Because the vaccine is less effective at that age, a baby who receives the measles vaccine before age 1 should later get two more doses under the regular schedule.

Can the measles vaccine cause side effects?

The measles vaccine is extremely safe. Some children — about 10 percent — will develop a fever about six to 12 days after vaccination, and an even smaller percentage will have a rash that can last a few hours to a day or two. In extremely rare cases, the fever can precipitate a seizure in children predisposed to fever-induced seizures. Scientific research has shown unequivocally that the measles vaccine does NOT cause autism or other permanent neurologic or developmental problems.

Can the measles vaccine cause full-blown infection?

The vaccine contains a live but weakened form of the measles virus that is designed to create immunity without causing full-blown illness. In children with normal immune systems, the vaccine will not cause full-blown measles.

A tiny percentage of children born with defective immune systems may develop a measles-like illness after receiving the vaccine. If the immune deficiency is diagnosed in time, these children should not be vaccinated. Because infants and a small number of other people cannot receive the vaccine, it is critically important that the rest of the population be vaccinated to prevent outbreaks and spread of the disease.

What should I do to protect myself and my family?

The best way to protect against measles is to get vaccinated.