Though many mold species can grow indoors, the list of usual suspects is much shorter—and one variety gets the lion's share of attention. Columbia University's Dr. Ginger Chew studies asthmas and allergies related to mold. "When people think of mold, they usually mention toxic black mold, or Stachybotrys chartarum," she notes. This home invader defends itself from rival microorganisms by releasing poisons called mycotoxins, which can cause serious illness in humans. "Mycotoxins tend to inhibit protein synthesis, which your body needs to survive," says Dr. Chew. "They can cause hemorrhaging in various body tissues."