Some people think they can only transmit herpes to their partner if they have an outbreak, or in the prodromal period immediately before and then again right after an outbreak. They're wrong. Genital herpes can be transmitted at any time which is why some individuals who have many sexual partners or whose partners are not infected with the herpes virus may decide to use suppressive therapy against HSV even though they do not have any symptoms.
Although herpes can be transmitted at any time, people with herpes are not equally infectious at all times. In general, a person is most infectious during an outbreak, in the period immediately before and after an outbreak, and in the first year after he is infected, but he still can transmit the virus at other times. Furthermore, some people don't recognize when they are having an outbreak. One study of 53 individuals who tested positive for genital herpes, but who had said they did not actually have outbreaks, found that half them actually were having outbreaks but didn't realize it. However, even with that complication aside, six out of seven of the individuals who had no signs of clinical herpes were still found to be shedding virus.
Unfortunately, many patients and many physicians, are unaware of both the risk of asymptomatic transmission of herpes and how effective suppressive treatment can be in preventing it. If you are living with herpes and have one or more uninfected sexual partners, consider discussing the possible advantages of daily suppressive therapy with your doctor. Condoms aren't 100% effective at preventing the spread of herpes, since it is transmitted from skin to skin. So remind him, or her, that 70% of new herpes infections are acquired from people who have no symptoms at the time of transmission and that suppressive therapy has been shown to reduce the likelihood of transmission by half or more.
Note:Suppressive therapy isn't 100 percent effective at eliminating asymptomatic shedding, particularly for people with frequent outbreaks, and it is no substitute for condom use. However, using drugs and safe sex together may be the best way to protect your partner from your genital herpes infection.
Wald et al. "Standard-dose and high-dose daily antiviral therapy for short episodes of genital HSV-2 reactivation: three randomised, open-label, cross-over trials." Lancet. Available online 4 January 2012
Corey L et al. "Once-daily valacyclovir to reduce the risk of transmission of genital herpes." N Engl J Med. 2004 Jan 1;350(1):11-20.
Romanowski B, Zdanowicz YM, Owens ST. "In search of optimal genital herpes management and standard of care (INSIGHTS): doctors' and patients' perceptions of genital herpes." Sex Transm Infect. 2008 Feb;84(1):51-6.
Wald, A. et al. "Reactivation of Genital Herpes Simplex Virus Type 2 Infection in Asymptomatic Seropositive Persons" N Engl J Med. 2000 342:844-850.