Human bodies are just too damn complex. Even when the problem is clearly identified--a lack of oxygen in the blood--there's no way to just insert more that doesn't cause a hundred new problems :-/ Natural selection seems to be really bad at modular design.
Or the other way around, modular design does not appear to be a requirement for "success.". We like it for the reduction in complexity but that is ultimately a tradeoff.
So putting someone on an oxygenator might condemn the patient to stay on one until the virus kills them.