When it comes to spider bites on humans, two kinds of spiders--the Brown Recluse and the Black Widow--are typically to blame.
Black Widows live in tree stumps, wood piles, and even campground portable toilets, and they are easy to identify by the reddish "hourglass" on their bellies. The spider's bite causes shooting pain and appears as two "dots" made by the fangs at the bite site. Nausea, increased blood pressure, and vomiting occur soon after and require immediate medical attention.
The Brown Recluse (shown at left) lives in closets and attics in the South and Midwest in the United States. Their bite is painless, but the wounds they cause are serious. The bites are often red, then white, and have blisters in the shape of a bullseye. These bites also require immediate medical attention.