A pandemic refers to a global outbreak of a disease that affects a large number of people across multiple countries or continents. This can lead to widespread illness, death, and disruption of normal societal functioning. Pandemics are typically caused by the rapid spread of a new infectious agent or pathogen that the population has little to no immunity to. Common examples of pandemics include the influenza pandemic of 1918, the HIV/AIDS pandemic, and the ongoing COVID-19 pandemic. Research in this area focuses on understanding the spread and impact of diseases, developing strategies for prevention and control, and improving public health responses to future pandemics.