more than one type i would guess would be something like: "he had various different athlete's foot infections" where "athlete's foot" would be an adjective describing the type of infection(s).
As for more than one foot being infected i still think it would be "athlete's foot", (e.g. "he has athlete's foot on both his feet")
More than one athlete with the infection would still be "athtlete's foot", (e.g., "they all have athletes foot")
multiple athletes? I would think it would be the same thing, (e.g., "they all have athlete's foot on both their feet", or "all of their feet have athlete's foot infections")
for multiple different infections on multiple althletes on both their feet, i would guess "they all have various strains of athlete's foot infections one or more of their feet". Then again i am not an english teacher (so i could be wrong), but like BronzeWing says here i cannot think of a sentence where you would use anything but "athlete's foot" or "athlete's foot infections". (yeah i spent to much of my day already thinking about this) -enlil | 