How To Treat Red Rashes Caused By Spider Bite?

Wife got bit by a small red spider on her leg ,at her work,,, she works at a Hospital in the PBX Dept, The bite area had a small black center and a red rash about the size of nickel, the other bite was just a round red rash about the same size.... After they rash went away in about a week, two weeks later the rash returned in the same place as the bites... She said it itched so back she felt like digging her nails into her skin to scratch it.. I had her put some Aloe Vera - I cut fresh off the plant, and it seemed to stop the itch and by the next day it had already looked better.....it still took 3-4 days for it to go away.......Later a coworker told her that on that same day, he had seen a bunch of little red spiders under some of that same desk.... What kind of spiders can these be and how bad is their venom , thanks,,,John Birlew, San Diego, CA