Pregnancy

A temporary, usually nonfatal condition involving a parasitic embryo, a host female, and a disinterested third party (usually a father). The embryo attaches itself to the inside of the females' uterus and fees off of her blood and other bodily fluids, growing very rapidly, much like a tapeworm, only rounder. Fortunately, the parasite usually detaches itself
(See: Kidney Stones for a description of the pain involved in this process.) within 8-9 months, after which its parasitic tendencies become largely financial in nature.
Fortunately, we now know that pregnancy is entirely preventable simply by staying home on Friday and Saturday nights.