Out of roughly 38,000 known spider species worldwide, only about 100 have venom that can harm humans. The vast majority of spiders either can’t pierce human skin or produce venom too mild to cause anything beyond brief, minor irritation. In North America, just two groups pose a real medical threat: widow spiders and recluse spiders.
Why Spiders Bite in the First Place
No spider seeks out humans to bite. Spiders are predators that use their venom to subdue insects and other small prey, not to attack people. Bites happen when a spider is accidentally trapped against skin (inside a shoe, under bedsheets, inside a glove) or when it mistakes a person’s movement for prey. This is why most bites occur on hands, feet, and arms rather than exposed skin a spider could easily see and avoid.
All spiders technically have fangs and venom glands. But most species have fangs too small and weak to break through human skin. Of the 650 spider species in the UK, for example, only about 12 can bite a person at all. The situation is similar elsewhere: the spiders you see in your home, on your porch, or in your garden are almost certainly harmless to you.
Black Widows: The Most Notorious Biters
Five species of widow spider live in North America, and the southern black widow is the most widespread and likely to encounter people. You can identify females by their glossy black body and the red hourglass marking on the underside of the abdomen. Males are smaller, lighter in color, and rarely bite.
Black widows build messy, irregular webs close to the ground in dark, sheltered spots: woodpiles, garages, sheds, crawl spaces, and outdoor toilets. They’re not aggressive, but they will bite if pressed against skin or disturbed in their web. The bite itself feels like a pinprick, and you’ll typically know something happened right away. Within an hour or two, the venom can cause intense muscle cramping (especially in the abdomen, back, and chest), sweating, nausea, and elevated blood pressure. These symptoms are painful but rarely life-threatening in healthy adults.
Brown Recluses: The Misidentified Spider
Brown recluse spiders get blamed for far more bites than they actually deliver. In the United States, they’re only found in the South, Southwest, and Midwest. Bites reported outside these regions are almost always something else entirely. Thirteen recluse species live in the U.S., but the brown recluse is the most common and medically relevant.
These spiders are light brown with a violin-shaped marking on their back. They hide in undisturbed places: closets, attics, storage boxes, and behind furniture. A brown recluse bite is painless at first. Three to eight hours later, the site turns red and feels like it’s burning. It may develop a bruised, bluish appearance or a bullseye pattern. In mild cases (when the spider injected little venom), the discomfort resolves within a few days. In severe cases, the skin breaks down into an ulcer over one to two weeks, and healing can take months. Some people also develop fever, chills, dizziness, and body aches.
The Misdiagnosis Problem
Many skin wounds blamed on brown recluse bites are actually staph infections, particularly MRSA. Doctors have noted that when they culture supposed “spider bites,” the results frequently come back as MRSA. A legitimate brown recluse bite diagnosis requires that someone actually saw the spider bite them or caught it at the scene. If you develop a painful, swollen skin lesion and didn’t see a spider, a bacterial infection is statistically more likely, especially if you live outside the recluse’s geographic range.
Other Spiders That Can Bite
Yellow sac spiders are the third most commonly cited biting spider in North America. Two species live on the continent, and they’re pale yellow-green, about the size of a nickel. They sometimes wander indoors at night and can bite if trapped in clothing or bedding. Their bites cause localized redness, swelling, and a burning sensation, but the effects are mild and resolve on their own.
Some tarantulas in Latin America, Africa, and Asia carry neurotoxins in their venom potent enough to cause serious reactions. North American tarantulas, by contrast, have relatively mild venom. Their bite is painful because of the large fangs, but the venom itself is comparable to a bee sting for most people. Tarantulas also defend themselves by flicking tiny barbed hairs from their abdomen, which can irritate skin and eyes.
Outside the Americas, the Australian funnel-web spider and the Brazilian wandering spider are among the most dangerous species. Both are aggressive relative to other spiders and carry potent neurotoxic venom. Antivenoms exist for both, and fatalities have become rare where medical care is accessible.
Spiders That Won’t Hurt You
The spiders you’re most likely to encounter indoors are virtually harmless. Cellar spiders (often called daddy longlegs) are the wispy, long-legged spiders that hang in corners and basements. Their fangs are too small to penetrate skin. Jumping spiders, the compact, big-eyed spiders you see on windowsills, can technically bite but almost never do, and the result is less painful than a mosquito bite. Orb weavers, the ones building classic round webs in gardens, are docile and avoid human contact.
House spiders, wolf spiders, and garden spiders all fall into the same category. They look alarming because of their size or speed, but their venom is designed for insects and poses no meaningful risk to people. Finding any of these in your home is not a cause for concern.
What to Do if You’re Bitten
For any spider bite, the first steps are simple. Wash the area with mild soap and water, then hold a cool, damp cloth against it for about 15 minutes to reduce swelling and pain. If the bite is on a hand or foot, keep it elevated. Over-the-counter pain relievers handle the discomfort, and an antihistamine or calamine lotion helps with itching.
Most spider bites heal like a minor insect sting within a day or two. You should get medical attention promptly if you develop severe pain or stomach cramping, if the wound grows larger or develops streaks of redness spreading outward, if you have trouble breathing or swallowing, or if the skin around the bite turns dark purple or brown. If you can safely capture the spider (even squished), bring it along. Identification makes treatment much more straightforward.
Keep in mind that the overwhelming majority of “spider bites” never involved a spider at all. If you didn’t see a spider, consider other explanations: insect stings, allergic reactions, or skin infections. A red, swollen bump that appeared overnight is far more likely from a mosquito, flea, or bed bug than from a spider that had no reason to bite you.