The Centers for Disease Control and Prevention (CDC) estimate there are more than 300,000 species of fungi in existence, including household molds. While there's neither the time nor need to list and describe them all, some species are more commonly found invading homes. At the very least, knowing about these common molds is worth your time.
Black pepper-like spots growing in cool, damp areas like your toilet are prime real estate for a genus of mold called Cladosporium. They usually thrive in summer where they can release up to 50,000 spores per cubic meter of air. Scientists estimate there are around 40 species of Cladosporium; among the most common include Cladosporium herbarum, the most common in the genus.
The CDC attributed Cladosporium to the outbreak of meningitis in several states in the East and a few in the West (thankfully, Arizona isn't included) in 2012. Its nature as an airborne mold has helped Cladosporium spread from state to state, city to city, home to home. Total elimination of this mold, according to mold removal experts, is nearly impossible.
However, like most molds, you can deny them of their most valuable resources like the cold, damp environment they thrive on. Keep the bathroom and other cold, damp areas ventilated. At the first sign of a Cladosporium colony, quickly respond with some bleach and hard scrubbing. The bleach will deprive them of much needed oxygen.