A latent herpes type 2 virus can wait inside the cells of the nervous system called neurons for days, months, or even years. At some future time, the virus awakens and causes the cell to produce thousands of new viruses which causes an active infection. Sometimes an active infection occurs without visible sores. Therefore, an infected person can spread herpes virus to other people even in the absence of sores.
A popular sign of the herpes virus waking up and becoming active again is an itch anywhere in the genital area. That is when it is most contagious and easily transmitted sexually.