Or you can do it in more elegant fashion by using setter function on health variable.
var health = 100 setget set_health
...
func set_health(value):
health = value
if value <= 0:
die()
...
func on_area_entered(area):
self.health -= 5. # It's very important to write self.health instead of health if you use setter function
...
func die():
$death_timer.start()
$AnimationPlayer.play("death")