Furthermore, we found that the decreased expression of DPP4 in the oral salivary glands of elderly individuals may be closely related to viral infection and side effects of dysbiosis of the oral microbiome by conducting gene enrichment analysis (Figure 8B) because DPP4-associated proteins were mainly enriched in biological processes such as regulation of receptor-mediated endocytosis of viruses by host cells and bacterial invasion of epithelial cells. The gene discussed is DPP4; the disease is viral infectious disease.