In epidemiology, an epidemic (epi- meaning "upon or above" and demic- meaning "people"), occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is "expected," based on recent experience (the number of new cases in the population during a specified period of time is called the "incidence rate"). (An epizootic is the analogous circumstance within an animal population.) In recent usages, the disease is not required to be communicable; examples include cancer or heart disease. Another example includes the infamous Black Death of the Middle Ages.
Defining an epidemic can be subjective, depending in part on what is "expected" -- see also Endemic (epidemiology). An epidemic may be restricted to one locale (an outbreak), more general (an "epidemic") or even global (pandemic). Because it is based on what is "expected" or thought normal, a few cases of a very rare disease may be classified as an "epidemic," while many cases of a common disease (such as the common cold) would not.
The term syndemic refers to interacting epidemics that increase the health burden of affected populations. Social conditions that heighten the health risk of populations (e.g. poverty, discrimination and stigmatization, and marginalization) by increasing stress, malnutrition, interpersonal violence, and the experience of deprivation, increase the clustering of epidemic diseases and the likelihood of their interacting.
The term "epidemic" is often used in a sense to refer to widespread and growing societal problems, for example, in discussions of obesity or drug addiction. It can also be used metaphorically to relate a type of problem like those mentioned above.
Factors that have been described by Mark Woolhouse and Sonya Gowtage-Sequeria to stimulate the rise of new epidemics  include:
1. Mar. 2009 - The Influenza A, aka the "H1N1" virus, a subtype of influenza virus A and the most common cause of influenza in humans.