HDV is an RNA virus that infects only in the presence of HBV, affecting about 13% of HBsAg carriers. In Italy, prevalence ranges from 3.2% to 9.3%. It increases the risk of cirrhosis, fulminant hepatitis, and HCC, particularly in high-risk groups (HIV, HCV, drug users, dialysis patients). Until 2020, pegIFN was the only therapy; since 2022, bulevirtide (BLV) has been available, blocking viral entry into hepatocytes and reducing HDV RNA and liver stiffness, with efficacy in 45-48% of patients, though the optimal treatment duration remains uncertain. The gut microbiota and bile acids also play a role in fibrosis and cirrhosis progression: dysbiosis, typical in cirrhotic patients, alters bile acid metabolism and increases intrahepatic toxicity.