Where do maggots that come out of dead bodies come from? One theory is that flies lay their eggs on the body and produce the maggots. Or, do they produce on their own in a dead body?
This was actually a point of great debate about 150 years ago or so. There are however some simple experiments you can do to resolve the issue. You take a sample of something that bacteria can grow in, say chicken stock, then you seal it off so that no bacteria can get in, you then heat it up to kill any that may be in there. If bacteria can appear from nothing then they will quickly infect the sample. However they can not, a fact you can be thankful next time you open up a can of some food. This was discovered by Prof Louis Pasture and we now of course call it “Pasteurization” in his honor.
My father is a biologist as is actually working on a book about the history of this.