what causes cat to die

[catlady12345]

There was a stray we have been feeding for a few months then one day it was drooling a lot and was sluggish. We found it dead a few hours later what could of caused the cat to die? The cat appeared healthy for months, the only other thing that occurred a few weeks earlier is the cat lost a patch of fur. I would guess the cat at about 3 yrs old.