We have drain flies in our basement... a lot of them... we have a toilet , a shower and double tubs down their as well as a floor drain... bleach and drano has not helped.. it is really grossing me out... prior to this the toilet was not used much and the shower not for a long time... is it possible the 100 year old pipes have shifted in the floor drain allowing the flies in?

The flies don't get in, they are born down inside the pipes. How to Get Rid of Drain Flies

I'm in my second infestation of drain flies. I've just about got them eliminated. The most important thing is to locate the source of the infestation and either seal it off, as I did, or use the method described in the link. Once you've eliminated the source you can began to kill the ones left in your house. Several fly traps are mentioned but I found that sticky fly strips kill the most. I hung mine over the kitchen sink drain after first placing pans over the drains to seal them off. I had already bombed both bathrooms, (the source was a bade wax seal in the master bath) and kept the doors closed to prevent flies from entering. I figure 8if I can just keep them out of the drains and from laying eggs they'll die off since drain flies have a life cycle of 5 or 6 days. So that's what I'm doing. Here's some more for you to read,
How to Get Rid of Drain Flies
Good luck, Tom Sorry Bob, I didn't realize we put up the same link,
Update! I tried both "vinegar in a Dish" fly traps. Left them out for 24 hours and NOTHING. However 3 of then flew into my open coffee cup and drowned.. Go figure!