Even though the thought of mushrooms growing in the bathroom seems a bit disgusting, ideal conditions exist for a mushroom to flourish. In the presence of moisture, high humidity, low temperatures and darkness, the mushroom can make its home in the bathroom.
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.
Mushrooms favour 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.
Mushrooms do not have chlorophyll and cannot utilise 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.