Any person that spends enough time in the outdoors will wake up one morning with a swollen hand or foot for no apparent reason. There are a number of spiders that can bite and can cause a rather nasty reaction when they do. Although the daddy long legs is the most poisonous spider you can handle, its mouth isn't big enough to grab on to a human. Black widow spiders, brown recluse and tarantulas are the most famous poisonous North American spiders you can encounter. The southwestern United States is also blessed to have a number of scorpion species, none that are fatally poisonous, unlike some of their African cousins which can pack a lethal punch.
Scorpion stings are also relatively common in the southwestern United States.
In more serious cases fever, nausea, vomiting and difficult breathing can follow. Some spiders like the brown recluse can cause serious tissue damage around the area bitten. When bitten by a spider, especially a brown recluse, tarantula or black widow, or when a scorpion stings you, you should seek out medical attention. Put a cold pack on the bite area. If the victim is having a serious reaction, make sure to keep the effected area below the heart.