First of all, it needs to be emphasized that occult chronic hepatitis B is not a new disease, it has only been recognized in recent years and is coming into people’s view more and more frequently. Previously, we only had the means to test for the five hepatitis B items, and our knowledge of hepatitis B was limited to serum HBsAg positivity, believing that only serum HBsAg positivity was the only evidence to confirm the diagnosis of hepatitis B virus infection; with the maturation of genetic testing technology, people have discovered a more accurate method to diagnose hepatitis B virus infection: HBV DNA testing. With the application of both tests, we have the finding that most patients with hepatitis B have positive serum HBsAg and HBV DNA, while a small number of patients with hepatitis B have positive serum and/or HBV DNA in liver tissues and cells, but negative serum HBsAg. These serum HBsAg-negative hepatitis B patients have been diagnosed with chronic hepatitis of unknown cause in the past. Based on the difference in test results, we call chronic hepatitis B with negative serum HBsAg and positive serum and/or intracellular HBV DNA in liver tissue and cells occult chronic hepatitis B. Why is there a serum HBsAg-negative hepatitis B virus infection? Or to ask another way: why can serum HBsAg not be positive after hepatitis B virus infection? On this point, we may consider HBV DNA as the entity of hepatitis B virus and HBsAg as the equivalent of a label attached to the surface of the entity of hepatitis B virus, which becomes unrecognizable or missing when this label is modified, covered, or torn off for some reason. In other words, despite the presence of the hepatitis B virus entity (HBV DNA), HBsAg is masked or missing due to the hepatitis B virus itself (e.g., virus mutation), or as a result of the interaction between the virus and the body, or for various other reasons that are unknown at this time, which results in: HBV DNA positive and HBsAg negative. The discovery of occult chronic hepatitis B has overturned much of the previous understanding of hepatitis B. It has been found that in people with occult chronic hepatitis B infection, HBV DNA can be detected in human serum, hepatocytes, peripheral blood individual nuclei, or other body cells. Occult chronic hepatitis B is often seen clinically in those who are negative for all five hepatitis B items, but also in those who are negative for serum HBV DNA and positive for HBV DNA in other body cells, and even in those who are positive for HBsAb. The possibility of occult chronic hepatitis B virus infection cannot be ruled out in patients with acute hepatitis B who were previously considered to have recovered (probably because the body’s immune system maintained the virus at very low levels and progressive liver lesions may still occur), healthy blood donors, and patients with chronic hepatitis B who have spontaneous or antiviral treatment with HBsAg conversion. Occult hepatitis B virus infection can also cause cirrhosis and hepatocellular carcinoma.