I thin our body sometimes 'knows' subconsciously what strengthens it and what does harm to it, so we may listen to it's 'inner voice' and allow it to act as it wants. But sometimes these mechanisms (homeostasis) don't work properly.
On the higher level of our consciousness we can decide what to do and what should be avoided (e.g. smoking, and so on). And we sometimes make better or worse decisions regarding our health.
On the other hand there are people who trust no one and try to cure every disease they have with their own home methods. Unfortunately it may lead to the point when not much can be done to help them.