I have a fairly recent construction, with hot water heat. I also have two humidifiers in the house to avoid electric shocks and nosebleads. Many of the modern evaporative units have a contained resevoir that only lets a small amount of water out at any 1 time. When used in conjunction with a microbicide, there is little chance of any problem from germs blooming. I did have that problem with the old belt and drum style that was designed late 60's, early 70's. The one in the bedroom is a Kenmore, which I think has too much water standing outside the tank to really recomend it. The main unit is a Bemis console, 2- 3 gallon tanks, with only about 1 qt. outside the tanks at any one time.
I saw an ears/nose/throat specialist recently, and he mentioned that while he saw a lot of people who wouldn't have a problem if they used a humidifier, he had NEVER seen a person who had been made sick from germs building up in the humidifier.
Since a REALLY RANCID mold appears to be the first thing that grows, I suspect there may be some truth to his statement.