Infectious Disease Epidemiology


Infectious Disease Epidemiology: 

Selected definitions 

1): Infection: The entry and development  or multiplication of a disease_ producing  agent in the body of man or animals.  An infection may or may not lead to a disease state. 

2): Infectious  disease: A clinically  manifest disease of man or animal resulting  from a infection.

3): Contagious Disease: A disease  that is transmitted  through contact e.g., STD, leprosy, scabies , tuberculosis

4): Infectation: For persons or animals the lodegement, development  and reproduction of arthropods on the  surface of the  body  or in the  clothing, e.g., lice, itch mite. Some authorities use  the term also to describe  invasion of the gut by parasitic worms, e.g., ascariasis.  

5): Host : A person or  other animal, including  birds and arthropods,  that affords sustenance or lodgement  to an infectious agent under natural conditions. An obligate host means the only host, e.g., man in measles and typhoid fever. Hosts  in which the parasite attains maturity  or passes its sexual stage are primary  or definitive hosts; those in which  the parasite is in its larval or asexual state are secondary  or intermediate  hosts. A transport host is a carrier in which the organism remains alive but does not undergo development.

6): Opportunitic infection: 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,  M. tuberculosis,  M. avium  intracellular, pneumocystic,  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. 

7): Iatrogenic ( physician  _ induced disease) : 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 of 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 anaemia following  the use of chloramphenicol, childhood  leukaemia  due to prenatal X_ rays, hepatitis__ Following  blood transfusion, etc. These are all preventable.In short, iatrogenic disease is a  hazard of health care

8): Epudemic :( Epi= upon;  demos= people). An outbreak of disease  in a community in excess of " normal expectation", and derived from a common epidemic diseases in India today  are measles,  chickenpox , viral hepatitis A, cholera,  enteric fever, and cerebro_ spinal meningitis. AIDS is a newer epidemic disease  worldwide. Several non_ communicable  disease  such as cancer, coronary heart disease,  hypertension  and traffic accidents  are occurring  in epidemic proportions in both developed  and developing  countries. 

9): Endemic :(En= in;  demos= people). The constant presence  of disease  within  a geographic  area, or the usual prevalence  of a given disease in a particular  area. The  endemic diseases common in India are enteric fever, diarrhoea and dysentery, viral hepatitis, leprosy, filaria, malaria,  tuberculosis,  etc. When conditions  are favourable, an endemic  disease  may flare up into an epidemic, e.g., typhoid fever, malaria, polio. 

10): Sporadic: This incidence  at intervals of single, scattered cases of disease, e.g., polio. A sporadic may be the starting point of an epidemic. 

11):  Pandemic : (Pan  = all; demos= people). An epidemic  which spreads from country  to country  or over the whole world, as for example the recent epidemic  of AIDS. Other  notable disease which have occurred in pandemic magnitude  in the past are influenza, cholera and plague. 

12): Zoonoses: Diseases or infections which are naturally  transmitted  between  vertebrate animals and man, e.g., rabies, plague, bovine tuberculosis,  anthrax.

13): Communicable  Diseases:  An illness  due to a specific  infectious  agent or its toxic products which, under certain conditions, tends to spread  among individuals  in a community.  

14): Non_ communicable Disease: This  term is applied to such diseases as cancer, cardiovascular  disease and diabetes which are not communicated from person to person. 

15): Inclubation period: This is the time interval between  the entry of the  disease agent into the body and the appearance of the first sign or symptoms  of the disease  in question. 

16): Isolation: The separation  of a person with infectious  disease  from contact with other human  beings, for the period of communicability. 

17): Carrier: A person who harbours the disease  agent without having any outward signs and symptoms. In other words, a carrier is an outwardly healthy  person who is capable  of infecting others. Carriers have been classified  as temporary   and chronic. A temporary  carrier is one who excretes the disease  agent only for short periods, e.g., cholera, dysentery, diphtheria.  A chronic carrier is one who  sheds the disease  agent for indefinite  periods, e.g., typhoid fever, amoebiasis. 

18): Antiserum: Serum containing  specific  antibody, e.g., ATS. 

19): Fomites: Inanimate  articles other than food or water contaminated  by the infectious   discharges from a patient, and capable  of harbouring  and transferring  the infectious agent to a healthy  person. Examples include handkerchief,  pencils, toys, utensils,  drinking  glasses.  

20) : Vector : Usually  are arthropod (e.g., mosquito) which transfers an infectious agent from an infected  person to a healthy host. 

21): Virulence: Measure  of the severity  of disease. 

22): Pathogenicity:  Ability to cause disease. 

23):  Nosocomial  Infection: 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  or the residual of an infection acquired  during a previous admission. It includes infections  acequired 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. 

24): Surveillance  : 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. 

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

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


While the  study of disease  distribution  and causation remains central  to epidemiology; the techniques of epidemiology  have a wider application  covering many more important  areas relating  not only to disease, but also health and health services. The uses of epidemiological are : 

1): To study historically the rise and fall of disease  in the population 

 It is well known that  the health and disease pattern in the community  is never constant. There are fluctuations  both over short and long periods of time. Epidemiology  provides  a means to study disease  profile and time trends in human population. By a study of these trends, we can make projections into the future and identify  emerging  health problem and their correlates. 

2):  Community  diagnosis 

 Community  diagnosis generally  refers to the identification  and qualification of health problems in a community  in terms of mortality and morbidity  rates and ratios, and identification  of their correlates for the purpose of  defining those individuals  or groups at risk, or those in need of health care. 

3): Planning  and evaluation  

Epidemiological  information  about the distribution  of health problems over time and place  provides the fundamental  basis for planning  and developing   the needed health services, and for assessing  the impact of these services on the people's  problems. 

4):   Evaluation  of individual's  risks and chancess

The  epidemiologists calculate relative risk and attributable risk for a factor related  to, or believed  to be cause of the disease.  The risk assessment  for smokers and non_ smokers,  for  selected  causes of death (
e.g., cancer and coronary  heart disease) is a well known example.  

5): Syndrome identification  

Epidemiological  investigations  can be used to define and refine syndromes.By observation of groups, such  studies have been able to correct misconceptions concerning  many disease syndromes. 

6): Completing the natural history of disease:

 The epidemiologists by studying  disease pattern in the community  in relation  to agent, host and environment factors is in a battle position to fill up the gaps in natural history of disease, than the clinicians. 

7): Searching   for causes and risk factors 

 Epidemiology  by relating  disease  to inter interpopulation differences and other attitudes  of the population  or cohorts examined, tries to identify  the  cause of disease. The concept  of " risk  factors " gave renewed  impetus to epidemiological  research. The search for causes and risk factors will be  a ceaseless  effort, as our ignorance  about disease aetiology, particularly  for chronic diseases, is profound, not to speak of  the " new" diseases  which are appearing. 


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