There are two spiders in the United States that are poisonous and can be found in and near areas frequented by people and pets. They are the Black Widow spider and the Brown Recluse spider. In both cases, these spiders are found in isolated areas away from regular human activity. Both of these venomous animals have a penchant for dark crevices, old rock piles, lumber stacks, trash and piles of long-unused clothing. They will be more common in rural parts of the country than in the cities and suburbs.
Unless you're in the habit of sticking your hands and feet into dark places, you are unlikely to encounter the black widow and brown recluse spiders.
Both the black widow and the brown recluse can kill people who are very young, very old or infirm. Initially, the black widow's bite is not much more than a pricking pain, but it soon becomes very painful, and the venom can make you thoroughly sick. Symptoms can be intense pain, a rigid, "boardlike" abdomen that mimics appendicitis, nausea, vomiting, convulsions, cold sweats, paralysis, breathing difficulties, discoloration of the skin, delirium and shock. The stiffened abdomen has almost been enough on occasion to land the victim of a black widow in surgery for an appendectomy. The brown recluse makes a thoroughly gruesome-looking lesion.
The victim of either spider should be rushed to a physician or hospital and, if possible, the spider or its remains should be brought along for identification. Ice packs on the bite will slow absorption of the venom. There is an antivenom for black widow bite, and if given soon enough, it will bring quick relief of symptoms. I haven't heard of an antivenom to counter the brown recluse's toxin, but medical treatments are available.