Some say this measles outbreak is a wake-up call and even bigger outbreaks lie ahead.
On Tuesday, a congressional committee looked at why vaccination rates are dropping and what can be done to bring them back up.
The committee heard from witnesses who said that in some wealthy areas of California vaccination rates are now the same as in developing nations.
The experts believe some parents are complacent because these diseases haven't been around for years.
They say other parents fear vaccines could be harmful.
The majority of current cases nationwide is among children.
But in California, nearly 60-percent are among adults.