There are some, like chlamydia, which can be asymptomatic (not show any symptoms) for a long time. The incidences of chlamydia have increased 150% in the past decade, and gonorrhea by 42%.
Recent surveys of young women attending GP clinics have found a chlamydia prevalence of 8.1% among those under 20 years old, and 5.2% among those 20-24 years old. However, rates vary widely according to the setting in which surveys take place.
For example, among women under 20 years old, surveys have found a chlamydia prevalence of 17.3% in GUM clinics, 12.6% in antenatal clinics, 12.3% in termination of pregnancy clinics, 10.7% in youth clinics, 10.0% in family planning clinics, and 5.0% in the general population. In each setting chlamydia prevalence is lower in higher age groups.4
http://www.avert.org/stdstatisticuk.htm