It destroys tissue and opens you up to secondary infections. There is a doctor in Texas who believes the toxins can linger unnoticed after the initial symptoms and hit you again later.
If I remember correctly, the brown recluse doesn’t have a venom per se. but injects an enzyme solution that sticks around and spreads slowly through the tissues.
I do remember reading an article that said that using electric shock across the bite area or the spreading necrotic flesh will disrupt the enzymes and stop the damage from spreading.
People carry stuff with them everywhere they travel, bedbugs, German roaches, pythons to the Glades, lot's of critters.
The official charts only show established breeding populations, individuals can show up anywhere.