A host of studies show that the relatives of patients with inflammatory disease are more likely to be ill themselves. For example, in a study of families who have a member with primary Sjogren’s syndrome, Anaya and team showed that 38% had at least one first-degree relative with an autoimmune disease versus 22% of control families.

While this clustering of inflammatory diseases in families could be explained by the sharing of common genes, such genes would confer no survival advantage and should subsequently have been weeded from the population. Instead, this clustering of diseases in families likely results from the sharing of common microbes. For example, the strains ofH. pylori that a child acquires reflect those present in his mother, father and even siblings.

Certain pathogens have been shown to cross the placental barrier and persist in the amniotic fluid, sperm and egg. These microbes can be passed easily from generation to generation. Pathogens acquired from the mother seem to have a particularly strong effect on the microbiome of subsequent generations. A number of studies now show that there is a microbiome in the placenta, which can impact a fetus during its time in the womb. Infants additionally acquire microbes from the vaginal canal during birth. Breast tissue and breast milk also harbor microbiomes that are passed to infants during feeding.