Because of this, a study last year that found that treating herpes did not actually lower people's risk of HIV confused many scientists. They could not understand why, even though the suppressive therapy used was moderately effective at reducing the number of herpes outbreaks, it didn't actually lower people's risk of getting HIV.
Fortunately, scientists think they may now have an answer. A small 2009 study discovered that the immune system cells that HIV prefers to infect are found in higher concentrations at the site of a herpes infection - for months after the herpes sores have healed. Even after successful treatment with a suppressive treatment like acylcovir, the increased concentrations of highly-susceptible cells remained present in the skin.
If these results are confirmed in further studies, it could explain both the increased susceptibility of individuals with herpes to HIV infection, even in the absence of symptoms, and why standard suppressive treatments may not be sufficient to reduce their risk.
Gupta R, Warren T, Wald A. "Genital herpes." Lancet. 2007 Dec 22;370(9605):2127-37.
Celum C et al. "Effect of aciclovir on HIV-1 acquisition in herpes simplex virus 2 seropositive women and men who have sex with men: a randomised, double-blind, placebo-controlled trial." Lancet. 2008 Jun 21;371(9630):2109-19.
Zhu, J. et al. "Persistence of HIV-1 receptor-positive cells after HSV-2 reactivation is a potential mechanism for increased HIV-1 acquisition." Nature Medicine. Published online: 02 August 2009 | doi:10.1038/nm.2006