Here is a great graph to show the impact of viral load on CD4 count. The two entities are nearly inverses and you can see on the graph that at month two (when most people do not know they are infected) the person has something called viremia where there is a huge amount of virus in the system. This is when someone would be most likely to transmit as they a) may not know their status and b) have a very high viral load.
The important thing to remember is treatment is prevention. According to the CDC:
The great majority of persons with HIV infection do not transmit HIV to others. CDC estimates that there were 5 transmissions per 100 persons living with HIV infection in the United States in 2006 [3]. This means that at least 95% of those living with HIV infection did not transmit the virus to others that year–an 89% decline in the estimated rate of HIV transmission since the peak level of new infections in the mid-1980s. The decline in transmission is likely due to effective prevention efforts and the availability of improved testing and treatments for HIV. The lower transmission rate is what has enabled HIV incidence to remain stable despite increasing prevalence [1].
The graph is via: http://sierrambulldog2015.wordpress.com/
hivnola 