PIERRE - Reported cases of the two most prevalent sexually transmitted diseases in South Dakota have decreased, the state Department of Health said.

Gonorrhea and chlamydia are the two most prevalent sexually transmitted diseases in the state, especially in people between the ages of 15 and 24, said Lon Kightlinger, state epidemiologist. He says chlamydia also is one of the most common infectious diseases, second only to influenza.

In 2007, the number of chlamydia cases dropped slightly - from 2,633 to 2,623. And gonorrhea cases decreased from 359 to 258.

Underage alcohol or drug use can lead to risky behaviors, contributing to the spread of STDs, according to Ruby Douglas, community prevention networker with Capital Area Counseling Service.

"Alcohol does play a big part in breaking down those inhibitions and making poor choices and decisions that can affect a person for a lifetime," Douglas said. "They're not making good decisions, and they may not be thinking of contracting STDs."