Syphilis cases have doubled in the U.S. since 2005, the Centers for Disease Control and Prevention said Thursday.
Last year, 16,663 were infected with the sexually transmitted disease, the highest rate in the U.S. since it was nearly eliminated in 2000.
The increase is seen almost entirely in men, who account for 91% of cases in the U.S., the CDC said. The highly contagious disease's infection rate among women has remained fairly constant since 2005.
In its report, the CDC added that new cases were most commonly diagnosed among men who have sex with men, cautioning that this is "a major public-health concern, particularly because syphilis and the behaviors associated with acquiring it increase the likelihood of acquiring and transmitting HIV."
If left untreated, syphilis can cause serious damage to the brain and nerves. The agency recommended that doctors encourage safer sex practices and increase screening for the STD.