enemy health

How do you make it so that the enemies health wont fall below zero?

Here.
Create a property named health. ( you did that already I guess)
Peoperty sensor [if health interval -9999 to 0 ] > property actuator assign property health “0”

Script:
If own.health < 0 :
own.health == 0

There are even more ways to do this, even without any python.It depends of what you do want to do.

If your weapon(s) all do the same damage (for example) “1” than its very easy: Set your health property (which is Integer) to “0” and if you do want your enemy die when it is hit 3 times you create the following bricks:
https://img.skitch.com/20120603-bggbcb995rfisgw4pxfqs74fqi.jpg
I usually dont call it health, but something like “hit counter” for example, because it adds/counts hits and does not subtract damage.
As I said, this works only if the enemys health can be divided by the players damage (eg. if damage is 1). If the damage is “2” then two hits would cause the health to 4 and then the sensor above wouldnt work anymore.

The other and more flexible way would be to use the Controller expression method. It is pretty the same thing like BlendingBGE said, but only without python scripting. So in your case it could look like this:
https://img.skitch.com/20120603-x33neqyxc9fcrd2un166kj4rj2.jpg
It simply states that the actuator will only be activated when this expression is positive. For example you could write the following:

Sensor Always Controller Expression: “Health<0” Actuator Property: Assign: “Health” to “0”
to make sure that health is never counted under 0

I hope that made it clear for you =)

Thank you blendingBGE it worked.I am trying to make a firstpersonshooter.I am making the ai first with logic bricks.

I thought enemies die when the health is zero or below?

If dead the enemy will not care what health he has anymore :evilgrin:

I am glad it did!