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Decades after the development of vaccines and antibiotics, infectious diseases have not disappeared as was predicted.
For the prevention and control of Infections, knowledge of its epidemiology is needed.
Definitions are critical for any type of epidemiological action e.g.
*Disease reporting,
*Measurement of mortality and morbidity

It’s defined as the entry and development or multiplication of an infectious agent in an organism including the body of man or animals.
It also implies that the body responds in some way to shield itself against the invader, either in the form of
*An immune response or
An infection does not always cause illness.

There are levels of infection:
1. Colonization, e.g. S. aureus in skin and normal nasopharynx
2. Subclinical or inapparent infection (e.g. Polio)
3. Latent infection (e.g. virus of herpes simplex) and
4. Manifest or clinical infection.

The presence of an infectious agent
*On a body surface
*On or in clothes, beddings, toys etc.
*On or in surgical instruments or dressings, or other inanimate articles or
*In substances like water, milk and food.
Pollution is distinct from contamination and denotes the presence of offensive but not necessarily infectious matter in the environment.
Contamination on a body surface does not suggest a carrier state

For persons or animals; ‘Infestation’ means:
The lodgment, Development and Reproduction of arthropods
*On the body surface or
*In the clothing
E.g., lice, itch mite
Infestation is also to describe invasion of the gut by parasitic worms e.g. ascariasis
For articles or premises: called as Infested when these harbor or give shelter to animal forms, particularly arthropods and rodents

A human, animal, bird or arthropod,
*That affords sustenance or lodgment to an infectious agent
*Under natural conditions (not experimental)
An obligate host is the only host for that infectious agent e.g. man in measles and typhoid fever.
In case of multiple hosts:
The host in which the parasite achieves maturity or undergoes its sexual stage is considered to be the primary or definitive host;
E.g. Anopheles mosquito for malarial parasite
The host in which the parasite exists in a larval or asexual stage is considered to be the secondary or intermediate hosts
E.g Man for malarial parasite

A transport host is a carrier in which the organism survives but does not undergo any kind of development
E.g. fishes which carry the plerocercoid of ‘broad fish tapeworm’ to larger fish which are eventually eaten by humans or other final hosts.

It’s a disease occurring due to an infectious agent.
All infectious diseases and infestations are communicable
Infectious/communicable diseases may be
*Contagious or

A Disease that spreads from person to person, animal to animal or between animals and people, by direct contact with the
*Infected individual or
*Their secretions
E.g. scabies, trachoma, STD and leprosy
All contagious diseases are infectious but not all infectious diseases are contagious
E.g. Food poisoning is infectious but not contagious

A disease occurring due to an infectious agent or its toxic products
And arises through transmission of that agent or the products
From an infected person, animal or reservoir
To a susceptible host,
*Either directly or
*Indirectly through an intermediate plant or animal host, vector or the non-living environment

In Greek, Epi = upon, demos = people
An epidemic is defined as the occurrence in a community or region
*Of cases of a disease, a health related behavior or a health-related event
*Clearly in excess of normal expectancy for that community/region
The community or region and the period in which the cases occur must be specified precisely.
The number of cases required to declare an epidemic varies according to
*The agent,
*Size and type of population exposed
*Previous exposure or lack of exposure to the disease; and
*Time and place of occurrence
Epidemicity is therefore comparative to the usual frequency of the disease in the same area, among the specified population, at the same season of the year. E.g.
Even a single case of a communicable disease not seen for long in a population or
A disease not previously reported in that area
Two cases of such disease associated in time and place may be sufficient to be considered an epidemic and
Requires immediate reporting and full field investigation
The purpose of surveillance systems is to detect epidemics at the earliest so that the control measures can be put underway.

In Greek; En=in/within and demos=people
It indicates a persisent presence of the particular disease or infectious agent
*Within a specified geographic area or population group
*In absence of any import from outside;
It may be considered as the "usual" or expected frequency of the disease within such area or population group.
E.g. common cold is endemic because somebody always has one.

“Hyperendemic" refers to persistent, high levels of disease occurrence.
The disease is constantly present at a high incidence and/or prevalence rate and affects all age groups equally

“Holoendemic“: A disease for which a high prevalence begins early in life and affects most of the child population,
*Thereby leading to a state of equilibrium such that
*The adults in a population show evidence of the disease much less commonly than do the children
This is seen with malaria in some geographical areas
An endemic disease can erupt into an epidemic when conditions are favorable (e.g., hepatitis A, typhoid fever)

Sporadic = scattered about or occurring at irregular intervals
The cases are reported irregularly, randomly, now and then, and usually uncommonly.
*The cases are infrequent and
*Disconnected with each other in space and time
*A common source of infection is not identifiable
E.g. tetanus, herpes-zoster and meningococcal meningitis
Sporadic cases may sometimes be the starting point of an epidemic.
Various zoonotic diseases are usually transmitted to man sporadically e.g. Rabies and tetanus

A pandemic is defined as “an epidemic occurring worldwide, or over a very wide area, crossing international boundaries and usually affecting a large number of people
It occurs when almost simultaneous transmission takes place worldwide.
To cause a pandemic, the infectious agent should not only be able to infect and cause disease in humans but also spread easily from human to human
In efforts to control the spread of the disease, there is often social turbulence and economic loss
Examples are COVID -19, influenza and cholera pandemics.

A disease that is not known to occur in a country but is imported into the country

Any disease or infection naturally transmissible from vertebrate animals to man
E.g. rabies, plague, bovine tuberculosis, anthrax, brucellosis, salmonellosis, endemic typhus, hydatidosis, kyasanur forest disease, monkeypox, lassa fever, etc.
Anthropozoonoses: are diseases transmissible from living animals to humans, e.g. rabies, plague, hydatid disease, anthrax
Zooanthroponoses: diseases that are mainly infections of humans but can be naturally transmitted to animals. e.g. human tuberculosis in cattle
Amphixenoses: The infectious agent is maintained in nature in both animal and human population and can be transmitted in both directions e.g. T cruzi, and S. japonicum

Denotes that the disease has high prevalence in a specified animal population at the specified time.
In other words, an outbreak of disease in an animal population (equivalent to an epidemic in human population)
Sometimes it poses a threat to human population too
E.g. avian influenza spread through the Eastern Mediterranean Region in 2006 with large epizootics in many countries while human infections were reported in Djibouti
Only a few zoonotic agents cause major epidemics in human population
E.g. anthrax, brucellosis, rabies, influenza, Rift valley fever, Q fever, Japanese encephalitis, equine encephalitis and Swine flu
The study of epizootic diseases is known as Epizootiology

An outbreak of a particular disease in a specific bird population

A disease is consistently present in a population of animals in a limited region, season or climate.
In other words, an endemic occurring in animals
E.g. , anthrax, rabies, brucellosis, bovine tuberculosis, endemic typhus and tick typhus.

NOSOCOMIAL INFECTION = Healthcare-associated infections (HAI) or hospital acquired infection
Infection acquired during the process of receiving health care
*In a hospital or other health care facility.
*It indicates a new illness, distinct from patient’s primary ailment
Provided, it was NOT
1. Present or incubating at the time of admission or
2. The residual of an infection acquired during a previous admission.
May appear even after discharge, and
*Include occupational infections that may affect staff
E.g. infection of surgical wounds, hepatitis B and urinary tract infections.

Result of a diagnostic or therapeutic procedure performed on a patient
The professional activity generates an adverse health effect.
Has been described as, “doctor-generated"
There are differing views on what constitutes iatrogenesis and its scope.
Medicine and public health are not the only professions that cause adverse health effects.
The disease may be serious enough to
*Prolong the hospital stay,
*Require special treatment or
*Actually threaten life.
Most are related to drug therapy, immunization or diagnostic procedures
*Reactions to penicillin and immunizing agents,
*Aplastic anemia following the use of chloramphenicol,
*Childhood leukemia due to prenatal X-rays,
*Hepatitis B following blood transfusion
These are all preventable.
In short, iatrogenic disease is a hazard of health care

Definition: Continuous Analysis, Interpretation, and Feedback of systematically collected data
Generally using methods distinguished by their practicality, uniformity, and rapidity Rather than by accuracy or completeness.
By observing trends in time, place and persons, changes can be detected or anticipated and appropriate action can be taken including investigative or control measures
Sources of data may relate directly to disease or to factors influencing disease.
Thus they may include mortality and morbidity reports based on:
Death Certificates,
Hospital Records,
General Practice Sentinels, Or
Laboratory Diagnosis;
Outbreak Reports;
Vaccine Uptake and Side Effects;
Sickness Absence Records;
Changes in Disease Agents, Vectors, or Reservoirs;
Serological surveillance through serum banks

Cessation of all transmission of the particular infection
*By extinction of the infectious agent
*Through surveillance and containment
Thus ‘Eradication’ is an absolute or ‘total’ process i.e. an "all or none"
To be used only in case of termination of an infection from the entire world.
It implies that disease will no longer occur in a population.
Till-date, the only disease that has been eradicated is smallpox.
The term elimination is sometimes used to refer to "eradication" of disease from a large geographic region or political jurisdiction, (e.g. measles)
As per the current knowledge, diseases amenable to eradication are
*Polio and
*Guinea worm

1. Park’s Textbook of Preventive and Social Medicine. 25th ed. 2019. M/ s BANARSIDAS BHANOT, JABALPUR
2. Merriam-webster website: available at:, accessed on 1st December 2020 at 2 PM.
3. WHO website: available at, accessed on 4th December 2020, at 1:50 PM
4. WHO website: available at, accessed on 5th December 2020, at 11:55 AM