Horrible smell in Downstairs bathroom

Have the in-laws been visiting?

Turds

Dump some water down the toilet also as the trap is dried out.

If there is a tub, the trap may have dried out. I don't use my tub opting to shower in the basement and I have to run water in the tub occasionally to refill the trap.

Toilets have a trap? Is this something new?

The trap is built into the toilet, not the waste line.

I know that but it wasn't obvious that the previous poster did.

And maybe should have worded my reply "toilets need a trap?"

I just put duct tape over a shower I never use. Alternately RV antifreeze

If all ideas of dry traps fail the toilet wax ring is bad

You may have a floor drain with a dry trap.

Run some water in the shower. The trap has dried out.

Bob

Yes, just take a look in it. See the water?

External trap as I perceived the other poster was alluding.

DID you have beans for dinner last night? :)

If you have a sink, tub, or other drain that has not been used, the trap is empty and sewer gasses are coming into the house. You must maintain water in all traps.

You said the toilet leaks. Where is the leak, if it's the wax ring on the floor, it only takes a small wind outside to push sewer gasses under that wax ring and into the house. Assuming the leak is just the wax ring, why dont you just replace it. It's not that difficult and then you can use the toilet. Even if the toilet is cracked, they are not all that costly if you just buy a cheap one, or find a good used one.

Are you on an ejector pump (overhead sewer) or is your main line below the basement floor level? If your on an ejector and you rarely use the basement drains, make sure the pump pit did not fill up due to pump failure. If there is water in toilet then that should be ok.

Which brings to mind a question. There was a rarely used building with bathrooms & on a sewer system. When we went in there after months of unuse, the commodes were empty & smelling. With the first flush there was always a moderate "boom". Never did figure out why it did that. Any ideas?

Red

Air trieing to get out of the pipe

The other poster was me. I was not alluding, I was stating a fact. No water, you get sewer stink. I probably should have mentioned to check all traps, not just the toilet.

No fact but rather a conjecture that the OP's toilet was dry.