Once the yeast get going, they will out compete other organisms for resources. The alcohol will kill off anything that is in there, which is why beer was essential back before modern water sanitation methods.
People thought Ben Franklin was insane for trying this crazy experiment where he drank only water. Everybody thought he was going to die. They used to brew table beers, which were 2-3% alcohol to kill off any microbes that may have been living in the water.
I live in a basement apartment that stays pretty damp in the summer and I have not had any issues DIY CO2. Now dishes that stay in the sink for more than half a day is another story.
I usually feel kind of guilty using the quick reply... my replies are rarely quick.