I am sure the doctor would be able to call on the latest scientific research to justify his conclusions that the nano-virus existed and caused the illness in question. But this is just a dry debate over conclusive or circumstantial evidence in the scientific or legal context.
Religion is concerned with matters of faith for which evidence is irrelevant, and for which the deliberate seeking of evidence wold in any case be a matter of apostasy, and specifically with the existence and actions of immortal beings on a "higher" plane of existence than our own, i.e. gods. Also with the question of what happens to human consciousness or the human soul after physical death of the body.
So not all matters taken on a degree of faith due to lack of conclusive evidence can be called a religion. But all systems of belief that form a framework or context for issues of world creation, deities, immortal beings, afterlife and reincarnation or progression of the soul after physical death to a "higher sphere" are religions.
No to use the existential evidence of certain phenomenons in a metaphysical sense as the criteria to define whether some thoughts constitutes religion or not is very limiting.
Anyway, I am confused as to why anyone would think buddhism is not a religion. What separates it from other major religious belief systems?
Get to know more about Buddhism and you will know why Buddhism is considered a school of thought and not really a religion in the sense as how we see religion in the conventional sense.