Denmark has registered a doubling of cases of the venereal disease chlamydia over the past decade, Danish health officials said Thursday, saying the rise was very worrying. \"In 2000 there were 15,000 registered cases (of chlamydia), and in 2009 the number had increased to 30,000,\" a report published by Denmark\'s National Board of Health said, adding that in addition, \"we think that one in 10 sexually active young people have the disease without knowing.\" If discovered in time, the bacterial disease can be treated with antibiotics, but going without treatment can be dangerous and around 500 infected people become sterile from it each year, the health board said. \"There is definitely cause for concern,\" said Bjarne Rasmussen, a medical consultant for the Frederiksberg Council in central Copenhagen who helped collect the statistics. \"Part of the increase in registered cases is a result of campaigns to get young people to be tested for the disease, but one of the main problems is that about half of those who are infected have no symptoms,\" he told AFP. \"The spread of the disease among young people is due to them not using condoms. They are good at protecting against pregnancy by using contraceptives, but (quickly) tend to drop condoms and go on to non-barrier contraception,\" Rasmussen explained. The health board and Frederiksberg Council have launched a large campaign to persuade young people aged 15 to 22 to be tested for the disease in order to get early treatment.