One doesn't always hear much about chlamydia, considering it's one of mankind's oldest and most infamous sexually transmitted diseases.

But chlamydia infects 3 million people every year in the United States, and new research suggests that it may be one of a rare group of infectious diseases that can cause cancer. 

Specifically, chlamydia drastically increases the risk of cervical cancer, according to a study in this week's Journal of the American Medical Association

The researchers, led by Dr. Tarja Anttila of Finland's National Public Health Institute, examined 128 women with advanced cervical cancer in Finland, Sweden and Norway. 

Compared to a control group of healthy women without chlamydia, women with the bacterial infection were up to 6.5 times more likely to develop cancer, depending on which strain of Chlamydia trachomatis they had. 

How a bacterial infection can produce cancer is still a bit of a mystery. But the human papilloma virus (HPV), which causes one type of genital wart, has long been known to cause cervical cancer by spurring cells in the cervix to grow out of control, and the chlamydia bacterium could do the same. 

Developing cancer from chlamydia "takes several years, probably decades," the Finnish researchers wrote; chlamydia often infects teenagers and people in their twenties. 

So why is chlamydia so often ignored? First, it lacks the drama of incurable viruses like HIV, HPV and herpes, since chlamydia is easily cured with antibiotics. More worrisome, most of the millions of people with it have no idea they're infected, which keeps the infection off the public's radar screens. 

Symptoms only turn up about 15 percent of the time for women and 60 percent of the time for men. But when there are symptoms, women experience vaginal bleeding, abdominal pain, painful intercourse and urination and abnormal or foul-smelling discharge. Men with chlamydia may experience discharge from the penis, painful or burning urination and sore testicles. 

Condoms will usually prevent the spread of chlamydia; if left untreated, it can cause sterility and other reproductive problems in both men and women. 

The bottom line: If you thought chlamydia was nothing to worry about, you were wrong.