OAKLAND -- A new analysis finds there were an estimated 1 million cases of sexually transmitted diseases among young Californians in 2005.

"It is clear we are living in the dark about how common STDs are and how many kids acquire them in a given year," said epidemiologist Petra Jerman of the Public Health Institute in Oakland, who led the study reported in the California Journal of Health Promotion.

The number of infections was not known before because many do not have to be reported to health authorities and those that are supposed to be reported often are not.

Jerman and her colleagues estimated incidence of STDs using a computer model developed by the U.S. Centers for Disease Control and Prevention. CDC researchers reported in 2004 that 9.1 million new STDs occurred nationwide among 15- to 24-year-olds in the year 2000.

Jerman and her colleagues estimated that there were 1.1 million new cases of STDs in California in that age group in 2005. They found the estimated cost of treating the infections was about $1 billion annually.

The number one infection reported in the study was human papillomavirus (HPV), at 590,000 cases followed by trichomoniasis with 250,000 cases.