Close encounters with bees and wasps are sometimes too close, and their stings do hurt, but for most of us, our contacts with these or other insects cause little more than annoyance. In a few instances, however, insects and other arthropods have tremendous impact on human life and history as carriers of deadly or very harmful diseases. Most spiders are not able to bite humans or do not have venoms, but a few spiders are dangerous and have killed people with their venoms.

The Bubonic Plague (or Black Death) was the worst disease epidemic in human history. It took 14 million lives--nearly 1 out of 4 people--in 14th-century Europe. The plague is passed to humans by the bite of the Oriental rat flea (Xenopsylla cheopis), which picks up the disease-causing bacteria from rats. The plague still occurs in various parts of the world--even in the United States!

Malaria kills hundreds of thousands of people each year. It is caused by protozoans that are transmitted by the Anopheles mosquito. While there are a few cases annually in North America, the threat of malaria is greatest in the tropics--where most anopheline mosquitoes live. Travelers can take precautions to reduce their risk of contracting malaria.

Lyme disease is caused by corkscrew-shaped bacteria carried by the black legged tick, Ixodes scapularis, in eastern United States, and a related tick, Ixodes pacificus, in western United States. The disease starts as a rash and can later involve the joints, nervous system, and heart. Treatment with antibiotics is sometimes effective.

The black widow spider (Latrodectus mactans) bites in self-defense. Its venom contains neurotoxins, which attack the nervous system, causing intense pain, nausea, and respiratory problems. Neurotoxins can be neutralized by an antivenin.

When it bites (always in self-defense), the brown recluse spider (Loxosceles reclusa) injects a venom with cytotoxins, which attack cell tissue. The resulting wound spreads slowly and can take months to heal unless treated with antibiotics and steroids.