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Important Term & Definition

A.    Infectious agent:  A microorganism that is capable of producing an infection. (bacteria, virus,   fungi, parasite, and prion)

B.     Infection:  Invasion of the body tissue of a host by an infectious agent whether or not it causes disease

  • Pathogenicity:  refer to the ability of microbial species to produce disease

  • Virulence is the ability of the strain of microorganisms to cause disease. M.  Tuberculosis contain a strain of a varying degree of virulence. Eg. The vaccine contains avirulent strain.

    • Virulence  of a strain is not constant and may undergo  variation

    • Exaltation:  enhancement of virulence of the strain of Microorganism

    • Attenuation:  reduction of virulence of a strain

  • Invasiveness: the ability of an organism to spread within the host tissue after establishing infection.









  • Type of infection:

    • Primary infection:  initial infection

    • Reinfection:  subsequent infection with the same parasite in the same host

    • Secondary infection:  when the body resistance is lower by preexisting infectious disease, a new parasite set up an infection  secondary infection

    • Cross infection:  when a patient already suffering from a disease acquires a new infection from another host for another external source. Ex Nosocominal infection

    • Iatrogenic infection:  Physician  induced infection resulting from drug therapy or an investigative procedure(infection like AIDS and Hepatitis B may sometimes be transmitted during a procedure such as injection, blood transfusion, dialysis, and surgery)

    • Subclinical infection:  when clinical symptoms of infection are not Apparent

    • Latent infection:  following infection,  some parasites  may remain in a latent  for hidden from in host tissue and they proliferate and produce disease when the resistance of the host is lowered

  • Atypical infection:  Atypical or characteristic clinical manifestations of the infectious disease are not present.

C.     Infectious disease: a disorder caused by  an infectious agent/organism

D.    Infestation:  invasion of insect and worm that causes a disease to the host. Eg.  Mites,  ticks, flea, lice & worm

a.      Ectoparasites:  on the surface of  host

b.      Endoparasite:   inside the host/  lumen of host

E.     Contagious disease:  diseases which are transmitted from one person to another person either by direct contact or by casual contact with their secretion/object touched by them

F.     Communicable diseases: a disease which is transmitted by direct and indirect (disease is spread from one person to another through a variety of ways)

G.    Endemic (स्थानिक):  a disease that exists in a particular/ restricted reason/ area for a population with relative Ni low level throughout the year

H.    Epidemic (महामारी):  quick and Rapid breathing of infectious disease on a mass scale

I.      Pandemic:  Global disease outbreak/ several countries/worldwide

J.      Exotic: a disease which is introduced from a foreign land

K.    Nosocomial infection:  infection acquired in Healthcare facility including Hospital biomedical intervention is called nosocomial infection or Healthcare-associated infection (HAI) { acquired as a clinical setting}

Example urinary tract infection &upper respiratory tract infection

L.     Host (get the disease):  a person or other(but on the living)  organism that is susceptible to or harbors an infectious agent under natural condition

a.      Primary host:  the host in which parasite residues  to sexual maturity/  primary host provides food shelter etc

b.      Intermediate host:  parasite friends only part of its life cycle or does not reach sexual maturity

M.   Reservoir (may be living or non-living):  Reservoir of an infectious agent is the habitat in which agents normally live, grows and multiplies to persist over a long period of time &May or may not show effect of illness

Eg:  environment (water, Soil,  plant),  human and animal

N.    Carriers:  an individual capable of transmitting pathogen/  agent without display symptoms is referred to as a carrier:

a.      Passive carriers: contaminated with agent & can mechanically transmit it to other post (the passive carrier is not infected)

b.      Active carriers: infected individual who can transmits disease to others

                i.     Incubatory: who can transmit the agent during the incubation before clinical illness begins

           ii.     Convalescent:  record from their illness but remain capable of transmitting to others

               iii.     Chronic: who continue to herbers agent/pathogen surcharge hepatitis B &Salmonella typhi

Mode of Disease's transmission


(the manner is which an agent is transmitted from its reservoir to a susceptible host

Vehicle (through Environments): Transmission of pathogen vehicles such as water, food, air.

 Mechanical vector transmission (without being infected):  carry a pathogen on its body from one host to another &  no development for multiplication of an infectious agent on or within the vector. 

 Biological vector transmission:  The pathogen reproduces within a biological vector that transmits the pathogen from one host to another &  infectious agent undergoes replication for development for both in vector.



Zoonosis is an infectious disease that is transmitted from vertebrate animals to humans under natural conditions.

Eg.  Brucellosis (cow and pig),  Anthrax (Sheep), plague (rodents),  trichinosis (swine),   rabies (dog, bats, caccons), HIV,Ebola, SARS

Disease transmitted by Arthropods as vectors


Arboviral disease (arthropod-borne viral disease)