The group of various Sac Spiders are known to deliver painful bites to humans. These spiders are not outright aggressive, but if threatened, they bite defensively. Because they are wanderers, they sometimes rest in a pile of clothes (clean or dirty) that have been left on the floor. When someone moves it, or puts on something from the pile, the Sac Spider may get trapped between the clothing and the person's skin and bite him/her to escape.
Like all spiders (except Cribellate Orbweavers), they produce a venom designed to immobilize their insect prey. While their venom is not seriously dangerous like a Brown Recluse's or Black Widow's, Sac Spider bites are slow to heal. It is not uncommon for the bite area to get infected as victims frequently scratch or touch it.