In 2000, measles had all but stopped spreading in the U.S. That's because vaccination rates were high enough to keep people from getting the disease.

When enough of the community is immunized against a contagious disease, most other members are protected from infection because there's little opportunity for the disease to spread.

But the virus spreads so easily that, when the first new measles case since 2000 was reported in 2005, it was linked to one unvaccinated U.S. resident. He was infected during a visit to Europe, which has lower vaccination rates.

The returning traveler infected American children who hadn't been vaccinated.

Luckily, a major outbreak didn't happen at that time. That's because enough people in surrounding communities were vaccinated against measles.

This type of protection is known as "community immunity" or "herd immunity."

When enough people in a community are protected against a contagious disease, it's hard for it to spread.

Newborns, pregnant women, or people with weak immune systems aren't able to get certain vaccines. Yet, with community immunity, even they will get some protection because the spread of a contagious disease is contained.

NIH MedlinePlus magazine talked to Anthony Fauci, M.D., director of NIH's National Institute of Allergy and Infectious Diseases. Dr. Fauci explained how vaccines work to protect us.

What is a vaccine?

Most vaccines are made from either a killed (inactivated) or live attenuated virus. The measles mumps rubella (MMR) vaccine is a live attenuated virus vaccine. This means the three viruses in the vaccine are not completely killed, but they are no longer able to cause the disease. They can alert your immune system to develop a response that can fight off these three diseases.

How is a vaccine developed?

After working on a potential vaccine in the lab, researchers start with a small Phase 1 clinical trial in healthy people to determine if the candidate vaccine is safe. If it proves to be safe, they then can move it into a larger Phase 2 clinical trial, which tests safety in more people and determines whether the candidate vaccine induces an immune response that likely will protect people from the virus, and helps us learn what the proper vaccine dosage is. After that, the potential vaccine can be moved into a large Phase 3 clinical trial, which tests safety in more people and determines whether the candidate vaccine can protect people against the disease.

Why do we wait to give the MMR vaccine to babies?

You generally do not want to vaccinate children before 1 year of age because their immune systems are not fully developed. However, if children between 6 and 11 months old are going to be traveling abroad or to an area in the United States with a measles outbreak, they should talk to their pediatrician about getting a dose of the vaccine before they travel. 

You usually want to start somewhere between 12 and 15 months for the first injection. Even then, not all babies respond, so you want to boost it with a second injection when they are between 4 and 6 years old. This will make sure they are fully protected against these three viruses.

MMR is one of the most effective vaccines available. It is about 97% effective when you give two doses.

Along with MedlinePlus, what resources would you recommend for someone who wants to learn more about vaccines?

One of the best resources for specific vaccine information is the Centers for Disease Control and Prevention. Visit CDC.gov to learn about all vaccines, including the MMR vaccine.

What advice do you have about vaccine safety?

My advice is that if you look at the science, as opposed to rumors or misinformation, it shows that the vaccines that are used now are very safe and highly effective, particularly the MMR vaccine.

Because measles is one of the most contagious viruses around and can sometimes have very bad outcomes, my message for parents is to get their children vaccinated against it and other vaccine-preventable diseases.