In the present chapter, we synthesize the epidemiological and mechanistic evidence linking individual viruses to PAH, map the virus-TLR-vessel interface, and assess translational implications: biomarkers (e.g., IFN-I/IL-6 signatures, circulating mtDNA), risk stratification, and therapeutic targets (i.e., antiviral therapies, TLR modulation, targeting the IL-6 axis, and restoring BMPR2 signaling) [35,37,180,181]. Here, IL6 is linked to pulmonary arterial hypertension.