Only a few spiders are dangerous to humans. Two that are present in the contiguous United States, and more common in the southern states, are the black widow and the brown recluse. Both spiders prefer warm climates and dark, dry places where flies are plentiful. They often live in dry, littered, undisturbed areas, such as closets, woodpiles and under sinks.

Black Widow
Although serious, a black widow bite is rarely lethal. You can identify this spider by the red hourglass marking on
its belly. The bite feels like a pinprick. You may not even know you've been bitten. At first you may notice slight
swelling and faint red marks. Within a few hours, though, intense pain and stiffness begin. Other signs and symptoms
include chills, fever, nausea and vomiting, and severe abdominal pain.

Brown Recluse
You can identify this spider by the violin-shaped marking on its back. The bite produces a mild stinging, followed by local redness and intense pain within eight hours. A fluid-filled blister forms at the site and then sloughs off
to leave a deep, enlarging ulcer. Reactions from a brown recluse spider bite vary from a mild fever and rash to nausea and listlessness. On rare occasions death results, more often in children.

If Bitten by a Black Widow or Brown Recluse
Cleanse the wound. Use soap and water to clean the wound and skin around the spider bite.
2. Slow the venom's spread. If the spider bite is on an arm or a leg, tie a snug bandage above the bite and elevate
the limb to help slow or halt the venom's spread. Ensure the bandage is not so tight that it cuts off circulation.
3. Use a cold cloth at the spider bite location. Apply a cloth dampened with cold water or filled with ice.
4. Seek immediate medical attention. Treatment for the bite of a black widow may require an anti-venom medication.
Doctors may treat a brown recluse spider bite with various medications.

-- Mayo Clinic