The gut microbiome appears to be increasingly responsible for at least some of the decline of the immune system during aging, and a new mouse study shows that it is reversible.

The gut microbiome

The microbiome describes a varied community of bacteria, archaea, eukarya, and viruses that inhabit our guts. The four bacterial phyla of Firmicutes, Bacteroidetes, Proteobacteria, and Actinobacteria comprise 98% of the intestinal microbiome.