Now a days, Everyone is talking about Epidemic, Pandemic. What are these terms? Today, I will talk about this.


(Epi means upon; demo means people). The “unusual” occurrence in a community or region of disease, specific health-related behavior (e.g. smoking) or other health-related events (e.g. traffic accidents) clearly in excess of “expected occurrence”. The amount of disease occurring in the past, in the absence of an epidemic, defines the “expected” frequency. Some use the term “outbreak” for a small, usually localized epidemic in the interest of
minimizing public alarm, unless the number of cases is indeed very large.

The above definition covers not only the usual epidemic diseases such as measles, chickenpox and cholera which are compressed in time, but also the modern “slow” epidemics of non-communicable diseases (e.g., CHO, lung cancer) in which the time scale of the epidemic is shifted from days or weeks to years (13). The slow growth of these epidemics conceal their size.

The key words in the definition of an epidemic are : in excess of “expected occurrence”. There is no agreement on what constitutes a significant excess. For example, in the US, a disease such as cholera is not normally present in the population. Therefore, even one case of cholera would constitute a “potential” epidemic in US. But in a country like India or Bangladesh, where cholera is always present in some population subgroups, a few hundred cases a year may be the “usual” or expected incidence (endemic situation). For cholera to be considered as an epidemic in India, hundreds of cases (i.e., cases above the endemic frequency) would have to occur. An arbitrary limit of two standard errors from the endemic frequency is used to define the epidemic threshold for common for diseases.



(En means in : Demo means people). It refers to the constant presence of a disease or infectious agent within a given geographic area or population group, without importation from outside; may also refer to the “usual” or expected frequency of the disease within such area or population group. For instance, common cold is endemic because somebody always has one.

The term “hyperendemic” expression that the disease is constantly present at a high incidence and/or prevalence rate and affects all age groups equally; and the term “holoendemic’ a high level of infection beginning early in life and affecting most of the child population, leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children, as in the case of malaria.

An endemic diseases when conditions are favorable may burst into epidemic (e.g., hepatitis A, typhoid fever). As new control or preventive measures are applied, the endemic status of a disease may change.


The word sporadic means scattered about. The cases occur irregularly, haphazardly from time to time, and generally infrequently. The cases are so few and separated widely in space and time that they show little or no connection with each other, nor a recognizable common source of infection, e.g., polio, tetanus, herpes-zoster and Meningococcal meningitis. A sporadic disease may be the starting point of an epidemic when conditions are favorable for its spread. Many zoonotic diseases are characterized by sporadic transmission to man.


An epidemic usually affecting a large proportion of the population, occurring over a wide geographic area such as section of a nation, a continent or the world e.g., influenza pandemics of 1918 and 1957, cholera El Tor in 1962 (still continuing) and acute hemorrhagic conjunctivitis in 1971 and 1981. We all know the example of Corona Virus, which has been categorized under Pandemic by W.H.O .


Diseases which are imported into a country in which they do not otherwise occur, as for example, rabies in UK. An example is the occurrence of epidemic polyarthritis in visitors to Fizi, due to Rose River virus (an alpha virus presumed to have been introduced by infected mosquitoes harbored in aircraft ).


An infection or infectious disease transmissible under natural conditions from vertebrate animals to man. May be enzootic or epizootic – e.g., rabies, plague, bovine tuberculosis, anthrax, brucellosis, salmonellosis, endemic typhus, hydatidosis, etc. In recent years several new zoonoses have emerged, e.g., Corona Virus.

The term zoonoses has been further amplified as follows :

(a) Anthropozoonoses  : infections transmitted to man from vertebrate animals, e.g., rabies, plague, hydatid disease, anthrax and trichinosis.

(b) Zooanthroponeses : infections transmitted from man to vertebrate animals, e.g., human tuberculosis in cattle.

(c) Amphixenoses : infections maintained in both man and lower vertebrate animals that may be transmitted in either direction, e.g., Trypanosoma cruzi, Schistosoma japonicum.


An outbreak (epidemic) of disease in an animal population (often with the implication that it may also affect human population). Only a few zoonotic agents of anthrax, brucellosis, rabies, influenza, Rift valley, Q fever, Japanese encephalitis and equine encephalitis. The study of epizoonotic diseases is given the name of epizzotiology.


An outbreak (epidemic) of disease in a bird population.


An endemic occurring in animals e.g., anthrax, rabies, brucellosis, bovine tuberculosis, endemic typhus and tick typhus.


Nosocomial (hospital acquired) infection is an infection originating in a patient while in a hospital or other health care facility . It denotes a new disorder (unrelated to the patient’s primary condition) associated with being in a hospital. That is, it was not present or incubating at the time of admission. It includes infection acquired during a previous admission or the residual of an infection acquired during a previous admission. It includes infections acquired in the hospital but appearing after discharge and also such infections among the staff of the facility. Examples include infection of surgical wounds, hepatitis B and urinary tract infections.


This is infection by an organism(s) that takes the opportunity provided by a defect in host defence to infect the host and hence cause disease. The organisms include Herpes simplex, Cytomegalovirus, Toxoplasma, Mycobacterium tuberculosis, Mycobacterium avium intracellulare, pneumocystis etc. (For example, opportunistic infections are very common in AIDS). Infection by an organism that is not normally pathogenic, but can cause disease if resistance is lowered.


Any untoward or adverse consequence of a preventive, diagnostic or therapeutic regimen or procedure, that causes impairment, handicap, disability or death resulting from a physician’s professional activity or from the professional activity of the other health professionals. The disease may be serious enough to prolong the hospital stay, require special treatment or actually threaten life. Most of the episodes are related to drug therapy, immunization or diagnostic procedures, e.g., reactions to penicillin and immunizing agents, aplastic anemia following the use of chloramphenicol, childhood leukemia due prenatal x-rays, hepatitis B following blood transfusion, etc. These are all preventable. In short, iatrogenic disease is a hazard of health care.


Surveillance has been defined as the continuous scrutiny of the factors that determine the occurrence and distribution of disease and other conditions of ill health. Surveillance is essential for effective control and prevention, and includes the collection, analysis, interpretation and distribution of relevant data for action.

Surveillance also connotes exercise of continuous scrutiny of health indices, nutritional status, and environment hazards, health practices and other factors that may affect health. Thus we epidemiological surveillance, serological surveillance, etc.

The main purpose of surveillance is to detect changes in trend or distribution in order to initiate investigative or control measures.


Termination of all transmission of infection by extermination of the infectious agent through surveillance and contaminants. Eradication is an absolute process, an “all or none” phenomenon, restricted to termination of an infection from the whole world. It implies that diseases will no longer in a population. To this date only one disease has been eradicated, that is smallpox.

The term elimination is sometimes used to describe “eradication” of disease (e.g., measles) from a large geographical region or political jurisdiction. In the state of our present knowledge, diseases which are amenable to eradicate are measles, diphtheria, polio and guinea worm and I also hope, this SARS-CoV Virus is going to be eradicated one day.

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