"Spider venom comes in many forms. It can often take a long while to discover the full effects of the bite. Naturalists have pondered this for years: there are spiders whose bite can cause the place bitten to rot and to die, sometimes more than a year after it was bitten. As to why spiders do this, the answer is simple. It's because spiders think this is funny, and they don't want you ever to forget them."
from Anansi Boys by Neil Gaiman