Genital Herpes infects approximately 1 in 5 sexually active people.  It is estimated that 90% of the people who have herpes are not aware that they are infected.  70% of the time when someone is infected it is during sex with someone who is having no symptoms.  It is caused by a virus similar to the Chicken Pox virus.  Once someone gets herpes, they are infected for the rest of their life, and may break out with blisters from time to time.  When someone has herpes, they can pass the infection on to others when they have blisters, but they may also pass the infection on to others when they have no symptoms.  It is possible to test for herpes by culturing the blister when someone has symptoms.  It is also possible to do a blood test to check for herpes during times when a person has no symptoms.

You can find more information about herpes on the CDC website.