The Definition of Widespread Disease

“Pandemic” is a specific term used by public health agencies to identify currently spreading and easily transmissible infectious diseases. Many in the general public use the term in a broader sense to indicate something that is general or universal, such as the “pandemic fear” of something (see Dictionary).

Pandemics are the later stage of epidemics, which stem from outbreaks – but not all outbreaks become epidemics, and not all epidemics become pandemics. These are all various steps of increasing magnitude for the spread of disease. One of the most common diseases to reach pandemic proportions is influenza, likely due to its constant presence in the human population and seasonal variations. The World Health Organization (WHO), the leading global public health agency, has defined the various stages based on influenza preparedness, as discussed below (see the Global Influenza Programme here).

How a Pandemic Starts: Outbreak

Only one disease has ever been wiped out from Earth – smallpox. All other diseases have some incidence throughout the year in some part of the world. Over the years, public health agencies have been able to track the numbers and expect a certain number of cases per year. In areas where a certain disease is expected, it is considered “endemic”. When the disease occurs somewhere it usually is not seen, or in a large number of people unexpectedly, it is considered an outbreak.

A Lack of Control

If an outbreak is not contained, the infection rate continues to increase and the disease continues to spread, it may become an epidemic. Generally speaking, an epidemic is a severe outbreak and has a similar definition – unexpected increased occurrence and/or spread of a disease (see Medterms). Some non-infectious diseases are considered to be epidemic – hypertension among African-Americans as an example. Infectious diseases that are not controlled at the epidemic level may spread globally, reaching the level of a pandemic (from the Greek: “all the people”).

The WHO Influenza Pandemic Phases

Because of the propensity for influenza A strains to become epidemic some years, the WHO devised pandemic phases in 1999 (revised in 2005 with actions revised in 2009) to aid in health agency preparation against pandemics. The most recent pandemic, 2009 H1N1, saw phase 6 in mid-2009 (CNN) and post-pandemic in August 2010 (CNN). Pandemic strains tend to also have increased morbidity and mortality associated with them. The phases devised for influenza are defined as follows:

Phase 1: Baseline virus circulation in animals with no known human infections.

Phase 2: A circulating animal virus has infected humans – potential pandemic threat.

Phase 3: An animal or reassortment virus has infected humans sporadically – limited human-to-human transmission but no community outbreaks.

Phase 4: Community outbreak, human-to-human transmission – significant risk of pandemic.

Phase 5: Epidemic, human-to-human spread in at least two countries in one WHO region – pandemic is considered imminent.

Phase 6: Pandemic, spread of the disease to a second WHO region.

Post-peak: Decreasing disease in affected areas, decreasing new infection rate. Second waves are possible with additional spread late in the pandemic.

Post-pandemic: Return to normal infection levels.