The potential harmful effects on human health attributed to spiders are of three kinds - allergic reactions, poison venom and fear.

Spiders may cause an allergic reaction, which occurs when people inhale the hairs or scales of spiders. This recognized allergic reaction is the same type as is caused by other arthropods such as cockroaches.

Spiders may inject venom through their bites in a process known as envenomization. While more than 50 species of spiders are known to be capable of biting humans, there are three - black widow, brown recluse and aggressive house spider - of greatest importance.

The black widow spider injects a neurotoxic venom that causes system symptoms with little or no local damage or necrosis. Symptoms of sweating, weakness, muscle pains, nausea, tremors, and vomiting can, in severe cases, create breathing difficulties, heart irregularities, and even death in those who are weak, such as the elderly and children.

Both the brown recluse and aggressive house spider inject cytotoxic venom which causes severe localized tissue damage and necrosis, but rarely systemic damage. Their bites involve skin loss and ulceration that can take months to heal and may require skin grafts.

Along with snakes, spiders are the most feared creatures on earth. People possess this common fear and in some instances will induce physical illness on seeing spiders.