This study investigates the pathogenesis of Severe Acute Respiratory Syndrome (SARS) by examining 18 autopsies of suspected SARS patients and analyzing white blood cells from 22 confirmed SARS patients. The research reveals that SARS virus can infect multiple cell types in various organs, including the respiratory tract, immune system, digestive tract, kidneys, and brain. The virus is detected in circulating lymphocytes, monocytes, and lymphoid tissues, as well as in epithelial cells of the respiratory tract, intestine, renal tubules, neurons, and macrophages. The study highlights the widespread dissemination of the virus and its impact on immune cells, particularly T lymphocytes, monocytes, and macrophages, which are key to the pathogenesis of SARS. The findings suggest that immune damage, more than lung damage, is the primary determinant of clinical outcome and mortality in SARS patients. The study proposes a comprehensive theory of SARS pathogenesis, emphasizing the importance of immune cell injury and pulmonary epithelial damage.This study investigates the pathogenesis of Severe Acute Respiratory Syndrome (SARS) by examining 18 autopsies of suspected SARS patients and analyzing white blood cells from 22 confirmed SARS patients. The research reveals that SARS virus can infect multiple cell types in various organs, including the respiratory tract, immune system, digestive tract, kidneys, and brain. The virus is detected in circulating lymphocytes, monocytes, and lymphoid tissues, as well as in epithelial cells of the respiratory tract, intestine, renal tubules, neurons, and macrophages. The study highlights the widespread dissemination of the virus and its impact on immune cells, particularly T lymphocytes, monocytes, and macrophages, which are key to the pathogenesis of SARS. The findings suggest that immune damage, more than lung damage, is the primary determinant of clinical outcome and mortality in SARS patients. The study proposes a comprehensive theory of SARS pathogenesis, emphasizing the importance of immune cell injury and pulmonary epithelial damage.