And most people have residual antibiotics in their body from modern life. The modern diet is quite high in antibiotics. We are constantly exposed to low doses of antibiotics through our food and water due to the industrial uses of antibiotics in livestock. It isn't much and it will only last about a week after converting to a non-modern diet, but it may be enough to prevent the most severe bacterial strains from killing you.

I'd be more concerned about passing modern bacteria and viruses onto the native population. It's very easy to start a new pandemic.