Usually, spouses will be eating the same diet; children mostly would be, but sometimes they won't eat this or that. Wider families may well eat many of the same foods. Even at the level of a country, there will be a lot of similarities between the diets of the majority of the population. And we already know that what we eat makes a considerable difference to our microbiomes, as do the medicines we take. So it would seem likely that the diet remains the main driver of many of these diseases. Plus, of course, related people will have many of the same genetic weaknesses; combine genes with diet and you get an explanation of why some diseases are more prevalent in some places, why some run in familes, why the incidence varies over time, and so on. The microbiome would be the mechanism rather than the cause.
But isn't it already known that even personality characteristics can sometimes be passed along with fecal transplants? I remember a recent article regarding this in Clinical Psychopharmacology and Neuroscience. Likewise there appear to be obesity linked and thinness linked microbiomes which are at least sometimes transmissible deliberately.
Further, there is no good reason it should be impossible for doctors to deliberately transmit cancer from one individual, or even a cancer lab culture, to a previously nondiseased individual. Historically, HIV has been deliberately infected by physicians, so why not cancer. I personally have miraculously escaped HIVi nfection from several deliberate physician attempts, although I haven't been tested since the last possible iatrogenic exposure at the beginning of this year, involving a 4 day period during which the virus could have been deliberately introduced by concomitant IV along with steroids.
Human beings, including doctors, are capable of horrors. There is no doubt in my mind that certain cancers lacking other explanation are frequently deliberately introduced by doctors. No doubt men have been deliberately infected with normally exclusively female cancers and vice versa. Such trust as I may once have had of doctors is long gone. I've had them try to kill me too many times. If you wish to call me paranoid, talk to the FBI about it. Doctors can be very bad; psychiatrists are generally very worse.