If the inside of your microwave is rusted can it harm your food?

In general, a bit of rust in the cavity of a microwave oven doesn't hurt things. It's a bit hard to clean around it, and it might be best to sand it down and spritz that place with a bit of paint to "seal" it from spatters. The only problem is if the rust is allowed to eat a hole in the cavity. That's bad. It can (and probably will) cause a microwave leak, and this may make the oven unsafe to use.

If you are uncomfortable taking a bit of emory cloth or the like to a rust spot, rubbing it down, and then spraying just a hint of paint (clear or any color) on the sanded area, ask a handyman to do it. It's not that big a deal. The paint will keep the rusted area from expanding and punching through the cavity.