Spiders do not transmit any diseases. Although nearly all spiders have venom glands, they rarely bite humans, and only a few species (like the black widow spider) can have a venomous -- but not usually fatal -- bite. Most spiders are nocturnal, shy, and avoid conflict by running away. They will only bite if they feel threatened (for example, being squeezed or held).
The degree of reaction to the black widow spider bite depends on the area of the body bitten, amount of venom injected, and a person's sensitivity to the venom. Serious long-term complications or death are very rare. If bitten, remain calm, and immediately get medical attention (contact your doctor, hospital, or poison control center). Apply an ice pack directly to the bite area to relieve swelling and pain.