Disease in an individual is an interaction between the individual (host), the disease-causing agent, and the environment. Each disease is unique in its causation and its effect on the individual. For example, the genetic make-up of an individual plays an important role in determining susceptibility and immunity of the individual when interacting with the disease agent and the environment. Exposure to a causative agent is a necessary step in the causation of disease in an individual. Disease could be caused by direct contact with another individual who has the disease or by contact with a common vehicle or vector (e.g., contaminated food or mosquito). For an individual, the risk of exposure to an agent could be related the distribution of the disease and the causative agent in community or population. Therefore, to understand the causation of a disease in a community, frequency of the disease and their determinants in the community or population need to be measured. By knowing the conditions in the community, health care providers can plan to deliver appropriate services. The disease burden in a community, and the evolution of that disease, helps to estimate the risk of disease development in the people in the community.

Incidence and prevalence are measures that help us to better understand the trend and patterns of disease occurrence. The most important step in determining these measures is to define the disease or what differentiates the individual as diseased or nondiseased.