Something that affects the structure or function of an organism is referred to as a
rash
disease
symptom

Modifier is something that affects the structure or function of an organism. Because modifier refers as modification, which means any changes of modification has done to change the state or structure of a certain organism or object.
For example:
=> Your dentist put braces to your teeth to make your teeth straighter. ( your teeth has been modified )
=> Your plastic surgeon did nose life to you ( your nose has been modified )

If something changes on a state of a matter, meaning it was being modified

Why  do you think body composition focuses  on the  ratio  of  fatty tissue  to  other body tissue? why is  body composition not related  to  weight?