Xplosis is right, except that with that technique, killing the zombie will also deactivate the timer.... therefore meaning you won't have to collect an anitode, jsut kill a zombie.
perhaps it would be better to do this:
in the setuplevel.fpi script, add the lines
:state=1,plrhealthless=450:timerstart
:state=1,timergreater=50000:plraddhealth=-10000
ok, so now make sure that a zombie does 51 damage in an attack.
this means that the timer is autonimous (separate) from the zombie, and therefore will continue even if the zombie is killed. your antidote must restore the players health to 500 for it to work.
with regards to
Quote: "Although, if I remember correctly it is quite hard to implement .gifs into FPSC"
I'd ignore it. It's innumerably hard to implement an animated gif into FPSC as the filetype isn't supported.