evidence: The variant, known as the "VB variant", causes CD4 cell decline to occur twice as fast in infected individuals compared with other viral variants. This is a clinical hallmark, or "signature" of the extent of damage caused by the HIV virus. In addition, those infected with the VB variant also demonstrated an increased risk of transmitting the virus to others, the data suggests. [...] This collection of mutations defines a particular type of virus, what we later came to call the VB variant (because it is virulent and part of the subtype B group of viruses). Technically, we performed a "principal components analysis": we found one direction in the space of all possible mutations that was correlated with viral load. Fifteen of the 17 individuals we found were from the Netherlands. To make sure this finding was not just a statistical fluke, we worked with colleagues at Stichting HIV Monitoring to obtain much more data from the Netherlands. We found another 92 individuals with the variant. Again, they had a substantially higher viral load than individuals with other HIV viruses, and we also found that their CD4 cells declined twice as fast. HIV attacks CD4 cells, gradually impairing an individual’s immune system, and the rate of this decline measures how much damage the virus is causing. [...] The VB variant of HIV provides a concrete example for the study of virulence evolution for viruses generally, which has been widely studied through theoretical means with few real-world examples to consider.