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 which in fact are not very common in urban, suburban and exurban areas. Both of these venomous animals have a penchant for dark crevices, old rock piles, lumber stacks, trash and piles of long-unused clothing.
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. I had a call from a Georgia physician not long ago who nearly performed such an operation after being misled by the sudden illness of a man who had been working in his barn. The operation was called off after the patient happened to mention that he had observed a number of spiders in the barn.
Both the black widow and the brown recluse can kill people who are very young, very old or infirm. 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 anti-venom for black widow bite, and if given soon enough, it will bring quick relief of symptoms. I haven't heard of an anti-venom to counter the brown recluse's toxin, but medical treatments are available.