Imagine that Wesker only wants people with a certain genetic characteristic A, which is always associated with another genetic characteristic B. Imagine that the B gene induces the production of a certain enzyme that doesn't allow the virus to consume your body (turning it into leeches/tentacles).
In that case, all the people with the B gene will survive, and all of those have the A gene, which is what Wesker is looking for.
If that is the case, then possibly, a person with these characteristics will not be consumed by the virus, even if an infected one tries to consume them, so the survivors wouldn't mind about the non-survivors. Of course, if this was Wesker's plan, the entire biosphere would be destroyed leading to the survivors' death, but maybe the virus' activity would stop some time after one is infected, so all the monsters would only be alive for some hours.
Of course, Wesker would be smart enough to chose a genetical characteristic he had himself (he wanted to be a god), so i'm 100% sure he was compatible with Uroboros (surely he knew his own genetic makeup).
Maybe he only got consumed by the virus (partially) because the concentration was too high, and his body didn't have time to produce enough "B-enzyme", but eventually, he was able to overpass it.