The science behind pandemics involves understanding how infectious diseases spread and impact populations on a global scale. Pandemics occur when a new **pathogen**—such as a virus or bacterium—emerges and spreads rapidly across countries or continents. Key factors include **transmissibility**, which determines how easily the pathogen spreads, and **infectiousness**, which measures its ability to cause illness. **Human-to-human transmission** can occur through various routes, such as **respiratory droplets** or **contact with contaminated surfaces**. **Host factors**, such as immunity and population density, influence the spread. **Epidemiological models** and **surveillance** help track outbreaks and inform **public health responses**, including **vaccination**, **quarantine**, and **social distancing** measures.