Thanks for commenting. I think you are probably referring to the study by Norton et al. (2010) (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2945313/), showing 6-times increased risk of dementia if his/her spouse also has dementia.

In the study, this association remains significant even after adjusting for common coufounders (age, sex, socioeconomic status, genetics, and education). So, the authors posited that "The distress of watching one’s spouse suffer from dementia, and the physical and mental burden of providing dementia care, are potential causal factors."

While the study did not examine the potential involvement of an infectious agent, I also think this is a possible factor explaining the association found, since partners tend to share similar microbiome profiles and infection risks.