Spider bites are uncommon, and many diagnosed spider bites are actually bites from other animals. Most spiders in the United States do not have fangs that are strong enough to penetrate human skin, or sufficient venom to cause serious harm. A bite from a non-poisonous spider may be pink, slightly raised, and have two visible puncture marks. Spider bites are usually solitary. A cluster of bites usually means the attacker was not a spider. Two poisonous spiders in the United States are the black widow and the brown recluse.

Black widow bites look like a pinprick. It takes 20 minutes to an hour before symptoms arise, but double fang marks may be immediately visible. Blood pressure and heart rate can increase soon after the bite. The area of the bite may increase in size and redness for several days. Black widow venom attacks the nervous system, and symptoms include cramps, stiffness, fever, and abdominal pain. Extreme cases include difficulty breathing and tremors.

Brown recluse bites vary in their severity. Mild cases are initially red, accompanied with small fang marks. The skin can become firm and discolored over the next few days, and eventually heal with little scarring. However, in severe cases, skin tissue is destroyed. This is known as necrosis. Brown recluse venom breaks down skin cells, and can cause open wounds that continue to grow. This begins with swelling, and the area turns dark red. Skin begins to slough off as it dies, and results in open lesions.