Final answer:
An outbreak refers to a situation where two or more people contract an infectious disease from a common source. This event can escalate to an epidemic or a global pandemic, depending on its spread and severity. Epidemiology studies the occurrence and spread of such diseases within populations.
Step-by-step explanation:
An outbreak refers to an event in which two or more people contract an infectious disease from a common source of infection. This can occur on different scales, ranging from localized epidemic to a widespread pandemic. The population density and mobility can dramatically impact the spread of a disease. For instance, an epidemic indicates a large-scale disease outbreak in a population and may signal a disruption in equilibrium regarding disease frequency, which could be due to environmental changes or changes in the population. On the other hand, an epidemic that becomes widespread over multiple countries or continents is termed a pandemic, as has been seen with HIV/AIDS and novel influenza strains.
In epidemiology, which is the study of patterns, causes, and effects of diseases in populations, diseases that suddenly increase in a specific area are identified as epidemic diseases. An example of such a disease is influenza, which typically sees a rise in cases during the winter months in the northern hemisphere. However, not every seasonal increase is considered an epidemic; it is the unexpected surge in cases that defines an epidemic event.
Diseases can also be endemic, meaning they are always present in the population at some level, or they can re-emerge when previously controlled. An example of a re-emerging bacterial zoonosis is brucellosis. Moreover, some diseases that affect humans are of zoonotic origin, caused by pathogens that are transmitted from animals to humans. Approximately 75 percent of recently emerging infectious diseases affecting humans are zoonotic.