Bad odor & cigaretter smoke from neighbor

I live in a two story townhouse in Palm Beach County FL. Most everyone in the community is a property owner, including myself and my immediate neighbors. Each building in the community has four townhouses attached forming a 't' or cross. There is this odd rotting smell that comes from one of the townhouses I share a wall with. Between having to breathe the couple’s cigarette smoke and now this foul smell, I don't know what to do. I called the HOA and they said that it was not possible that I could be smelling anything from a neighbor because the dividing walls were concrete block from top to bottom. We do not have an attic, but where the a/c filter gets changed on the ceiling of the top floor, I am able to see my a/c ducts and I am able to clearly see a wide opening into that neighbor’s a/c ducts (their above ceiling open area). This is definitely where most of their odors carry over into my home. Another area where the smell comes in is where there is plumbing along the shared wall. And as if that wasn’t enough the odors also blow in when the a/c is on, and since I live in Florida heat, that would be often. I find myself not wanting to be in my home, I don’t know what to do to solve this dilemma. One of the other neighbors that also shares a wall with them has also complained to our HOA and got the same result, nothing! The HOA has washed their hands and say they can't do anything. Please advice. Thank you kindly.

Its likely something in your walls or ventilation system. Either start looking in all the vents with a camera on a stick or hire someone to do it for you.

Start knocking on doors, make sure all your neighbors are alive. It is florida after all.

Wet drywall can smell like that, too.

I leave my furnace fan on ON all the time, year round. This prevents a musty smell from any stagnent air in the home.