Why is black mold dangerous to have in your household?

Controversial topic. The data on "black mold" is very controversial. There is data that says it is a problem and data that says it is not. In general, the presence of mold suggests a water or moisture problem that needs to be addressed.
Black Mold. Stachybotrys is the mold you are thinking of. The term "toxic mold" is not accurate. While certain molds are toxigenic, meaning they can produce toxins (specifically mycotoxins), the molds themselves are not toxic, or poisonous. This link will provide all the info and real facts. http://www.cdc.gov/mold/stachy.htm.