Have you ever heard of a pocket of brown fungus-y stuff growing inside a callus.
  No. 
I forgot to say earlier that I do a couple of small tow callous, that are always there and that bother me when feet are moist inside of shows for 6 hours or so, and when boot width is too tight causing one toe to rub against other.
Put a ban-aid around it and then no problems.
Ive had four moderate pus infections in my life an none were related to a callous.
Two were ticks { one two weeks ago },  one 15 years ago in crotch and it was a real problem til it go so large I popped it then all got better,
 recently  inside upper lip and never had one of those before so peroxide and lemon juice,
 and one from a been sting in wrist when I was around 6 years old.
Two staff infections  --summer time heat and not cleaning myself enough--- on nose and head of penis.