The book begins with an introduction to the virology of SARS-CoV-2 and its transmission, then systematically explores COVID-19 symptoms categorized by severity and affected organ systems. It dedicates a significant portion to Long COVID, examining its prevalence and potential mechanisms. Finally, it analyzes the impact of different variants on disease severity and vaccine effectiveness. This approach, drawing upon scientific literature and real-world data, provides healthcare professionals, public health officials, and researchers with a valuable resource for understanding and managing the disease and its long-term sequelae.