When case numbers rise sharply above the expected baseline, public health takes notice. An epidemic is the occurrence of a disease in a community or region at a level clearly in excess of normal expectancy.

Epidemics may be caused by infection, but the term also applies to non-infectious conditions such as opioid overdoses when they cluster in time and place. Investigation identifies the source, mode of spread, and population at risk. Control measures include isolation, vaccination, vector control, or behavioral interventions depending on the agent.

Timely surveillance and transparent reporting allow early containment. An epidemic that crosses international boundaries and affects many countries may be declared a pandemic. Historical epidemics have reshaped societies; modern tools give better odds of limiting damage.

• Disease occurrence clearly above expected levels
• Defined by time, place, and population
• Triggers investigation and control measures
• May remain local or grow into a pandemic