Most spiders don't bite humans and only do so if they're provoked. When spiders bite, the vast majority of bites only cause a mild reaction in people, such as minor swelling, inflammation, or itching.
However, there are a few venomous spiders in North America that can be dangerous. They are mostly found in the southern regions of the United States, but black widow spiders can also be found in southern regions of Canada. The venom that these spiders use to kill their prey is also harmful to humans. Being bitten by a venomous spider may cause severe local and systemic reactions and can sometimes be fatal.