The graph above shows the incidence and prevalence of AIDS in the U.S. from 1986 to 2003.

                    # of new cases of disease
Incidence =       # of people at risk

                   Total # of cases (old & new)
Prevalence =     # of people at risk

Epidemiology is the study of where disease occurs and how it spreads within populations.


Epidemiological studies involve the gathering of data related to a disease and analysis of the trends seen in this information.  Data is reported in the form of tables, graphs and charts that allow patterns to become visible.

Occurence can be categorized not only by frequency, but also geographic distribution.

Endemic diseases occur continuously in localized populations at a relatively low but steady frequency, such as malaria along the gulf and southern east coasts of the U.S., Western Africa, Central and South America, India, Sri Lanka and Southeast Asia.  These areas all have favorable climatic conditions to support the Anopheles mosquitoes that serve as vectors for the several Plasmodium species responsible for the disease.  The trypanosomes Trypanosoma gambiense and T. rhodesiense, agents of African trypanosomaisis (African sleeping sickness), are endemic to parts of Africa that support their tse-tse fly vectors, while Trypanosoma cruzi, the agent of American trypanomaisis (Chagas' disease), is endemic to North, Central and South America since these regions are home to the "cone-nosed" reduvid bugs of the genus Triatoma that are its vectors.

Sporatic diseases occur infrequently and may be scattered throughout populations and geographic locations.  When SARS appeared in the Americas in the outbreak of 2003, it appeared in a few cases in Toronto, Canada and a few in some U.S. cities, but then disappeared.

Epidemic diseases appear within a population, then rapidly spread outward.  For a disease to be considered epidemic, the observed number of cases must significantly exceed the recorded frequency of the disease based on regular case reporting.  In the graph (left), while disease A has a higher incidence than B, two epidemic outbreaks are seen in B since the number of cases in each exceeds expected frequencies.

If epidemics of the same disease occur within different countries at the same time, the disease is considered to be a pandemic.  Examples of pandemic diseases include the 1918-1919 outbreak of influenza H1N1 and the current AIDS pandemic.

In order to be able to classify the category of disease, it is necessary to provide specific information about incidence.  This data is gathered by case reporting performed routinely by infectious disease specialists at the local, state, federal and international levels.  Data about the most important pathogenic diseases is used to generate a list of notifiable diseases (above), reported on a weekly basis to the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia.