DAVID: What you call intelligence is the presence of alternative metabolic pathways. If one is not working the organism, the next is switched on, nothing more.

dhw: What switches it on, and why is it switched on in some but not in others if your God made the mechanism automatic?

Organisms have the ability to use whatever metabolic pathways are available within them. With species variability, not all organisms in the species have the same pathways onboard, which is why some are partially or fully immune/resistant and some are not. And remember horizontal gene transfer is also present primarily in bacteria.