Apparently you can get really, really sick if you don't keep your portable humidifier clean.
Here are Mayo Clinic's Tips for portable humidifiers:
Use distilled or demineralized water. Tap water contains minerals that can create deposits inside your humidifier that promote bacterial growth. And, when released into the air, these minerals often appear as white dust on your furniture. You may also breathe in some minerals that are dispersed into the air. In addition, use demineralization cartridges or filters if recommended by the manufacturer.
Clean humidifiers every three days. Unplug the humidifier before you clean it. Remove any mineral deposits or film from the tank or other parts of the humidifier with a 3 percent hydrogen peroxide solution, which is available at pharmacies. Some manufacturers recommend using chlorine bleach or other disinfectants.
Keep the area around humidifiers dry. If the area around a humidifier becomes damp or wet — including windows, carpeting, drapes or tablecloths — turn the humidifier down or reduce how frequently you use it.
Consider replacing your humidifier each season. Over time, your humidifier can build up deposits that are difficult or impossible to remove and encourage growth of bacteria. Even if you plan to toss out your humidifier at the end of the season, it still needs frequent cleaning in the meantime.
To keep humidifiers free of harmful mold, fungi and bacteria, follow the guidelines recommended by the manufacturer.
Buy one from Allegro Medical that needs very little maintenance, like the 3.5 Gallon Care Free Humidifier Plus
with 100% protection against bateria, molds and spores.
For more info, see Humidifier Types & Tips