There is no trigger or script that will definately keep a unit alive. Ther is always the possibility of a one hit kill, ie head shot or explosion. These will always kill the unit no matter what activation the trigger is set to, or time a looped script cycles.
IF you want to try your best to keep a unit alive then you can use a script and cycle it every 0.0000000001 seconds. This still wont stop a one shot kill.