How it kills: Unwanted but not uncommon house guests due to their size (less than 1/3 inch, at their longest), black widows build their loose, haphazard webs in dark places: the corners of rooms, under couches, and in basements or garages. Look out for adult females—brush up against a web the wrong way, and they'll bite, leaving behind a trail of cramping, nausea, respiratory pain, and possible paralysis at your expense.
How to get it before it gets you: Learn to identify a widow (shiny and dark; females have a reddish-orange hourglass mark on their abdomen), and periodically inspect the home hot spots listed above for webs while wearing gloves. Increase illumination in corners with artificial lights and by arranging furniture accordingly. Prevention is best, but vacuuming a widow and its web is safer than trying to crush it with a book, magazine, or hand.