The Centers for Disease Control and Prevention (CDC) believes measles—a highly contagious virus that causes skin rashes—may be making a comeback. Most years average 65 cases, according to the CDC, but 131 cases have already been reported in the first seven months of 2008. What’s the cause? Children who are not vaccinated because of parents’ religious views or principles, say the experts.

“Even with our current extremely high vaccination rates, we could have significant pockets of people in outbreaks,” says Jane Seward, MB, MPH, deputy director of the CDC’s viral disease division. “And if vaccine coverage levels fall, we risk many more outbreaks. We are concerned.”

While easy to treat, measles can lead to more serious life-threatening illnesses, such as encephalitis or pneumonia. Learn more about measles and vaccinations here.