Spores

• Spores are released from the gills of a mushroom and become airborne.mushroom in sepia image by Stephen VanHorn from Fotolia.com
  
  Mushroom spores exist on clothes, in hair and under fingernails. A good scrubbing and shower will generally wash them down the drain, or so you thought. One sticky spore is all it takes to root in the bathroom drain. Spores can become airborne on the mist from the shower and land anywhere.
  
  

Humidity

• Mushrooms favor moist environments, and bathrooms are the perfect place for them to flourish. Every time a person takes a shower, water creates a fine mist that clings to any surface. Often the bathroom door is closed after the shower, thus locking in the moisture that mushrooms love.
  
  

Nutrients

• Mushrooms do not have chlorophyll and cannot utilize sunlight to create energy; instead, they get nutrients from compost or dirt that washes down the bathroom sink or shower drain. Mycelia or slimes put forth by mushrooms capture the dirt and enzymes and convert them to usable carbohydrates.