Yes. Fluids mix all the damn time. It means nothing. I also think the "scratch theory" was only early guesswork by confused characters trying to come to terms with the puzzle. No one has died because of scratches. (Jim was bitten on his lower body.) But everyone is probably scratched a lot. They pin down zombies, bull rush them, etc. With all the close calls, I'm sure there's been a few overlooked scratches that haven't mattered. But the cross contamination of fluids happens all the time and it is purposefully ignored.
There is no disease. The rules for contamination don't reflect an infection. Occam's Razor states that the simplest explanation is the right one. So if virus can only be explained by adding crazy twists to the plot, it's probably wrong. Hence, magic, the craziest theory, is probably the right one.