in reply to The plural of "athlete's foot" is...

'Athlete's foot' has no plural -- there is only one condition called 'Athlete's Foot' -- thus it is never ever used in a plural sense.

"He has Athlete's Foot"
"They have Athlete's Foot"
"There were 17 cases of Athlete's Foot"

See, 'Athlete's foot' isn't a real noun and isn't a real adjective. It's about halfway between. It's the description of a condition.

"His {situation} was {dire}"
"His {condition} was {Athlete's Foot}"

However the plural form of these is interesting:

"Their {situation} was {dire}"
"Their {condition} was {Athlete's Foot}"

Huh? Now how come we don't pluralise when we're talking about more than one person? Because we're still only talking about one situation and one condition. If they all were in different situations we could have 'Their situations were dire', however there is only one condition that can be described as 'Athlete's foot' and thus the only way we can have more than one condition on the second line is to say 'Their conditions were Athlete's Foot and Measles'..

Now, while we're talking about plurals, how about 'Court Martial'?

It's 'Courts Martial'. See, it's not the martial of which there is more than one, but the court. Martial is the adjective that describes the noun.