
How old do you think Zeno is?
In comparison to Wesker who was 49 when he died in 2009, there's no shot Zeno was the same age at the same time.
If this were the case, he'd be 66 now, which would make him WAY more susceptible to the T-Virus in comparison to Leon, who got his final stages in less than 2 days. I think he's only faring well because of his abilities.
Even after he lost his powers, he still fared decently, because someone that old would just get slammed to the ground by a tackle from Leon.
This can also be backed up by the fact that we know Zeno was an artificial clone of Wesker made to copy Wesker, most likely AFTER they found out Wesker was a superior being, so when his powers fully manifested.
He's also a chain-smoker.