Can you rigorously define what 'natural to the body' means? It seems to me like an unfixed goalpost of sorts. Anything you like can be arbitrarily defined as 'natural' while anything you dislike is 'unnatural'. What differentiates your definition from the No True Scotsman Fallacy?
I don't see what is difficult to understand. Humans and their ancestors have breathed air into their lungs for millions of years. Therefore air in the lungs is natural to humans.
Corroboration from multiple unconnected sources constitutes evidence.
So then, Elvis is alive? Jesus and the Virgin Mary do appear to people all over the world? Aliens have landed on Earth?
You should rethink your standard of evidence, because it is not as strong as you think it is.
Those stories often are not corroborated. Someone seeing the Virgin Mary appearing to them over a spring cannot be corroborated. However, someone dying of cancer, given months to live, and then being cured after drinking the water from a spring can be corroborated. The story is corroborated by the caretakers, family members, and reporters around them who witness the recovery.
Some of those corroborators are more unconnected than others, such as the reporters who interview the patient, their doctors, and write up their story about the spring.
When multiple events like these happen, it just strengthens the evidence that the spring has some sort of property to it that inhibits or kills disease.
So you believe random people on the interwebs who tell you that vitamin c cures cancer, but you won't believe the space man who tells you that the earth is round? Got it.
Multiple corroborating reports of the earth being round
is evidence. If multiple sources are claiming that experience, with no contradicting evidence, it should be accepted as truth.
However, significant evidence of deception and fraud by the sources can call the claims into question.