My father was bitten by a spider on the left side of his forehead in 1989 while visiting America. The type of spider was never identified but the swelling was severe (almost baseball sized). He recovered with no ill effects.
However each summer, around the anniversary of the bite, in extremely hot weather, the bite location will swell and become red/inflamed.
He has never sought medical care for the swelling but it has remained a mystery/puzzle.
In the 90's he was resigned to never knowing why but I thought I would pose the question to SE to finally shed some light on the issue.
The swelling has not returned since approximately 2002-3 and appears that whatever caused it has dissipated but I have wondered for a long time what would cause a decade long cycle of swelling?