You can measure any physiological performance you want, but if you have a virus that targets red blood cells and targets the heart and targets the skeletal muscle, a simple hypothesis is that swim performance would be challenged. A simple test would be that they wouldn't be able to get to a maximum oxygen consumption. Those are the sorts of things that we do.
It's a line of least resistance. We could go and investigate PRV until the cows come home and I'd be long retired by then.