I'm not sure I buy that. Or, well, I suppose that those in the medical field believe it, but I don't think they're right.

Consider something like a surgeon nicking and artery while performing some routine surgery, the patient not responding normally to anesthesia or the anesthetist not getting the mixture right and the patient not coming back the way they went in. Or that subset of patients that have poor responses to a vaccine.

Everybody likes to think of the world as a linear system, but it's not.