This document defines disease and classifies major types as infectious, deficiency, hereditary, and physiological. It then focuses on three global diseases: HIV/AIDS, cancer, and coronavirus. For each, it discusses causes, symptoms, prevention methods, and how they spread. HIV/AIDS damages the immune system and can lead to AIDS. Cancer is caused by abnormal cell growth. Coronaviruses cause illnesses like COVID-19 with symptoms like fever and cough. Prevention strategies for all three include avoiding risk factors, practicing good hygiene, and social distancing. The document also briefly mentions some other diseases.