Why is there a terrible smell in my basement bathroom (mainly around commode) when the shower is turned on, on the main floor?

Why is there a terrible smell in my basement bathroom (mainly around commode) when the shower is turned on, on the main floor?

Do you use these bath room fixtures often in basement bathroom? If not your drain traps could be void of water (Evaporation). Simply flush comode and run water in each sink drain as well as bath tub and shower. This will fill traps with water and seal off sewer gases from the drains of your fixtures.

Could be a bad wax ring. Water running through the drains puts enough pressure to force air out a bad seal. Stuff some wet tissue paper around the toilet base and then run the shower. Smell go away? If so, bad wax ring.

Has it always been like this or just recently? I am going with option #3, bad ring and a venting issue.