An epidemic is a broad term used to describe any problem that is actively spreading and has grown out of control.

The pandemic relates to geographic spread. It describes a disease that affects a whole country or the entire world.

Epidemiology is the branch of medicine that handles the following:

• Incidence: the occurrence of a disease over a specified period.
• Prevalence: how many people are affected within a population.
• Control of diseases: an appropriate public health response.

Two measurable factors mostly define the level of disease occurrence:

• The pattern and speed by which a disease moves.
• The size of the susceptible population.

• Sporadic refers to a disease that occurs infrequently or irregularly.
• Cluster refers to a disease that occurs in larger numbers even though the actual number or cause may be uncertain.
• Endemic refers to the constant presence and/or general prevalence of a disease in a geographic population.
• Hyperendemic refers to persistent, high levels of disease well above what is seen in other populations.
• Epidemic refers to a sudden increase in the number of cases of a disease above what is normally expected.
• Outbreak is the same as an epidemic but is often used to describe a more limited geographic event.
• Pandemic refers to an epidemic that has spread over several countries or continents, usually affecting a large number of people.

The one staging model used to direct the public health response involves the flu. The same basic model can be applied with variations to epidemics like tuberculosis or malaria.

• Phases 1 through 3 help public health officials know it is time to develop tools and action plans to respond to an impending threat.
• Phase 4 through 6 happen when action plans are implemented in coordination with the WHO.