Arachnidism is a medical condition, simply defined as envenomation

Envenomation

Envenomation is the process by which venom is injected into some animal by the bite of a venomous animal. Many kinds of animals, including mammals , reptiles , spiders , insects , employ venom for hunting and for self defense...

 by a spider bite

Spider bite

A spider bite is an injury resulting from the bites of spiders or other closely related arachnids.Spiders are active hunters and rely heavily on their bites to paralyze and kill their prey before consuming it. They also bite in self defense...

, which, depending on the species, will have various effects. Bites from the widow spiders (genus Latrodectus) produce a condition known as Latrodectism

Latrodectism

Latrodectism is the clinical syndrome caused by the neurotoxic venom , that can be injected by the bite of any spider that is a member of the spider genus Latrodectus, in the family Theridiidae....

, while bites from the recluse spider

Recluse spider

The recluse spiders or brown spiders , also known as fiddle-back, violin spiders or reapers, are a venomous genus of spiders known for their necrotic bite, which sometimes produces a characteristic set of symptoms known as Loxoscelism...

s (genus Loxosceles) cause Loxoscelism

Loxoscelism

thumb|Scars left over from a healed recluse biteLoxoscelism is a condition produced by the bite of the recluse spiders . It is the only proven cause of arachnogenic necrosis in humans. While there is no known therapy effective for loxoscelism, there has been research on potential antivenoms and...

. Another medical condition that has historically been claimed to arise from spider bites is Tarantism

Tarantism

Tarantism is an alleged, possibly deadly envenomation, popularly believed to result from the bite of a kind of wolf spider called a "tarantula"...

, although there is very scant research supporting this supposed condition.