INTRODUCTION

A disease is any change in a state of normal health in which part or the whole body of an individual does not function properly due to the presence of an infectious agent or its product. It may be associated with specific symptoms and signs.
A disease is of various types, caused by
- external factors such as a pathogen and shared from one person to another called as infectious disease
- not caused by a pathogen and cannot be shared from one person to another called as non infectious disease
- caused by internal dysfunctions, such as autoimmune disease.

Epidemiology

Epidemiology (epi means upon, demos means population, logy means study) is the science that deals with occurrence, determination, distribution & control of a disease. An individual who studies the epidemiology is called epidemiologist.
- when a disease occurs occasionally & at regular intervals in a human population its known as sporadic disease eg. typhoid
- a disease maintaining a steady, low level frequency at a regular interval is called endemic disease eg. common cold
- a sudden increase in occurrence of a disease beyond a limit is called epidemic
- an occurrence of a disease that increases with a large population over a wide range is called pandemic.

Frequency of a Disease

Frequency of a disease is the repeated occurrence as fractions in a given population. It is related to morbidity and mortality.

Morbidity rate is the proportion of patients with a particular disease during a given year per given unit of population
Mortality rate is a measure of the number of deaths (in general, or due to a specific cause) in some population, scaled to the size of that population, per unit time.