What type of doctors treat herpes?

Your safest bet is probably a dermatologist, but if you've been formally diagnosed with it, you can probably see a general practicioner.  Since herpes has no cure, there really is no "treatment" for herpes aside from Valtrex and other supressive medications and safe, careful sex, so that you don't spread it.  The only part of that equation that a doctor can help with is the medication.  So if you've been tested and are positive, you can tell any doctor that, and he/she should be able to write you a prescription for it.  The rest is up to YOU.

P.S. - Doctors can also prescribe different topical medicines to help minimize outbreak times and the pain or itching associated with it, but again, any doctor can probably help you with this.