Comment by mjhay

It doesn't have to be mutually exclusive. Metabolic syndrome causes all sorts of general havoc with just about everything you can think of, which would probably include neuron's ability to maintain homeostasis or fight a latent infection.

Most everyone (at least older people who never got a chickenpox vax) has a latent varicella zoster (the shingles/chickenpox virus) infection, but only a minority will get Alzheimer's. It could easily be true that varicella is causal in most Alzheimer's, but also that many of those cases would never have happened without compounding risk factors like metabolic syndrome.