Okay, here's how I'd do this! First I'd create an object to represent the character.
<<set $skinnyHighSchoolKid to {}>>
Next I'd give him a variable that tracked his strength from 1 (low) to 10 (high). He'd start at 1.
<<set $skinnyHighSchoolKid.strength to 1>>
When he worked out, I'd increase his strength by 1, allowing it to get to a maximum of 10.
<<set $skinnyHighSchoolKid.strength += 1>>
<<if $skinnyHighSchoolKid.strength gt 10>>
<<set $skinnyHighSchoolKid.strength to 10>>
<</if>>
Each time I wanted to check if the character got sick, I'd create a random number from 1-10, and if the number were higher than his strength, I would set a new variable that made him sick.
<<set _sicknessChance to random(1,10)>>
<<if _sicknessChance gt $skinnyHighSchoolKid.strength>>
<<set $skinnyHighSchoolKid.sick to true>>
<</if>>
Finally I'd use <<if>> statements to make the sickness have an effect in the game.
Amanda invites you to the beach to play volleyball.
<<if $skinnyHighSchoolKid.sick>>
But you're too sick to go, so you [[stay home]] instead.
<<else>>
You can [[go to the beach with Amanda]] or [[stay home]].
<</if>>
To recover from his sickness, we'd just change the variable.
<<set $skinnyHighSchoolKid.sick to false>> You're feeling better now.