Epidemic is an outbreak of disease that attacks many people at about the same time. An epidemic may last a few hours, a few weeks, or many years. When a disease exists permanently in a region, it is said to be endemic. A disease that spreads throughout the world is pandemic. The term epidemic traditionally has been used to describe outbreaks of infectious diseases, such as measles, cholera, and influenza. People now often use the term to describe increased incidences of such noninfectious diseases as lung cancer or heart disease. Scientists called epidemiologists study epidemics to understand their causes and to stop epidemics from spreading.