Epidemology is the science that evaluates things that affect health and disease in seom defined human population. This is a practical science in that it attempts to figure out how disease is spread so that further spread can be prevented. In order to understand epidemology it must be noted exactly what a disease is. A disease is any situation where health is impaired. Helath is the normal status and function for a person, be it physical, emotional, or mental.

Epidemology arose in response to the great epidemics of the past. As stated above, it is a practical science. The father of epidemology, John Snow, figured out how cholera was spread (through contaminated water). He did this long before the pathogen was discovered, and this is the first instance of epdemolical investigation.

Before going any further, there are a good number of epidemologic terms that are used. A sporadic disease is one that occurs occasionally and a low levels in the general population. There is also no particular pattern to the individuals affected. A endemic is a disease that is present at low levels in a population most of the time, such as the common cold. A hyperendemic is when an endemic disease periodically increases in occurance, such as when the incidence of colds increases in the winter time. An epidemic is the sudden increase in incidence of a disease above the expected. Common colds are not an example here, as generally scientists know when they will hit. An outbreak is also a sudden and unexpected increase in incidence of a disease, but it occurs in only a limited subset of the population. Examples of this include food poisoning from a single source. Finally, a pandemic is an increase of a disease in a large population, with the example of worldwide influenza. Other terms include zoonoses, which are animal diseases that can infect humans. Morbidity is sickness and mortality is death.

Like any science, Epidemology needs tools to work with. A lot of these are mathematical. Epidemology is a science that involves a large amount of data collection and statistics, and the tools reflect this. The morbidity rate is calculated as the number of new cases per unit time divided by the number of individuals in the population. Morbidity therefore deals with new cases and in general the ability of a disease to be spread. Mortality rate is the number of deaths in a population in a given time divided by the number of infected individuals in the population. This deals with the lethality of the disease. The two are linked in part through the prevalence rate, which is the number of cases divided by the population at any time. The prevelance rate depends on three things: the incidence of new cases, the duration of the illness, and the lethality of the illness. It provides the basis for classifying diseases as epidemics.