> The team found that levels of two human herpes viruses, HHV-6 and HHV-7, were up to twice as high in brain tissue from people with Alzheimer's.

Yes, HHV-6 is common throughout the population, including in the brain. It's an endogenous retrovirus, so is inherited via DNA, or is easily transmitted from person to person, particularly in childhood.

The issue is severity, not presence.

By the way, the variants of herpes in question are HHV-6/7, which are different to the herpes simplex viruses (HSV1/2) that cause cold sores and skin breakouts.

craftyguy truncated the comment. "...common in the population between..." those who do and those who do not. I was talking about the water and breathing comment, clearly meaning that both the diseased and the unaffected have the behaviors of drinking water and breathing air in common. It's not absolute frequency that matters, but a difference in frequency between the diseased and the disease-free that is potentially explanatory.