So if the virus needs blood, lets say that it needs RBCs, then one route the virus might take in the short term is to increase EPO Erythropoietin production in its host. This would increase the number of RBCs in the host and as a side effect increasing the host athletic ability. In reality the differences might be subtle for somebody who isn't pushing their muscles' max oxygen requirements but certainly this could be used to help justify an increased stamina and greater tolerance of low oxygen levels in the infected people. Sustained increased EPO production leads to kidney damage and high blood pressure.
So is this just going to infect newts and frogs or will it infect people as well?