 -- California health officials are warning parents not to hold "measles parties" that intentionally expose their children to measles and other childhood diseases.

The California Department of Public Health "strongly recommends against the intentional exposure of children to measles as it unnecessarily places the exposed children at potentially grave risk and could contribute to further spread of the outbreak," the agency said in a statement today.

Measles parties were popular in the 1950s and 1960s before the MMR vaccine program was introduced for measles, mumps and rubella. The practice of measles parties is based on the belief that infected children will build up immunity to the virus because once someone has the measles they cannot catch it again.

But the U.S. Centers for Disease Control and Prevention also strongly discourages intentional measles exposure. The virus is particularly dangerous for children under five. In a worst-case scenario it can cause pneumonia, brain swelling or death.

Dr. Richard Besser, ABC News chief health and medical editor, echoed the same concerns about measles parties.

“This is a really bad idea,” he said. “Although most children recover from chicken pox and measles without a problem, not all do. The vaccines are far safer than the diseases.”

The CDC released new information earlier today revealing there are now 121 confirmed measles cases in 17 states since Jan. 1. Health officials believe the most recent outbreak originated earlier this year with group of unvaccinated people at Disneyland in California.

Besser cautioned that measles is a highly contagious disease that can linger in the air for up to two hours after exposure. Symptoms include high fever, runny nose, pink eye, cough and rash. People who are infected are contagious up to four days before symptoms appear and four days afterward.