It's very complicated.
Reality is often more complicated than any theory.
Modern medicine is finding this out in more than one arena. Sometimes several factors have to be in confluence in order for an effect to take place. The maddening search for the exact mechanism of “mad cow disease” is one such conundrum today. Just about everybody believes that prions, a kind of oddly folded protein, are in the middle of it, and yet what else interacts with the prions is less clear. A prion contamination does not guarantee getting “mad cow disease” though it empirically raises the odds. There are four or five main theories competing for acceptance.