Black widow bites

Latrotoxin is the poison that is in a black widow spider's bite.

Specifically, the arachnid in question (Lactrodectus spp) carries venom which functions as a neurotoxin. Latrotoxin therefore impacts the human system for 3-7 days in milder cases and in 21+ days in more extreme cases. The most extreme reaction will be death, on the way to which there will be abdominal cramps, extreme sweating, muscle pain, and sustained readings of high blood pressure and heart beats.