1. Important Terms/Definitions & Mode of Disease Transmission

Chapter 01: Important Terms/Definitions & Mode of Disease Transmission

1. Important terms:

2. Mode of transmission/Transmission of Disease


  1. Infectious agent:  a microorganism which is capable of producing an infection. (bacteria, virus,   fungi, parasite and prion)
  2. 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 contains 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  strain of Microorganism
Attenuation:  reduction of virulence of the strain
Invasiveness: the ability of an organism to spread within the host tissue after establishing infection.

Type of Infection:

Primary Infection
Secondary infection
cross infection
Itrogenic infection
Sub-clinical infection
Latent infection
Atypical 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 Nosocomial 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 form 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.
Type of Infection
Infestation, Ectopatasites, Endoparasites, Contagious disease, Communicable disease, Endemic, Epidemic & Pandemic disease, Nosocomial infection
  • Infestation:  invasion of insect and worm that causes a disease to the host. Eg.  Mites,  ticks, flea, lice & worm
    • Ectoparasites:  on the surface of a host.
    • Endoparasite:   inside the host/  lumen of host
  • 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
  • Communicable diseases: a disease which is transmitted by direct and indirect (disease is spread from one person to another through a variety of ways)
  • Endemic (स्थानिक):  a disease that exists in a particular/ restricted reason/ area for a population with relative Ni low level throughout the year
  • Epidemic (महामारी):  quick and Rapid breathing of infectious disease on a mass scale
  • Pandemic:  Global disease outbreak/ several countries/worldwide
  • Exotic: a disease which is introduced from a foreign land
  • Nosocomial infection:  infection acquired in Healthcare facility including Hospital biomedical intervention are called nosocomial infection or Healthcare-associated infection (HAI) { acquired as clinical setting}. Example urinary tract infection &upper respiratory tract infection

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
    • Primary host:  the host in which parasite residues  to sexual maturity/  primary host provides food shelter etc
    • Intermediate host:  parasite friends only part of its life cycle or does not reach sexual maturity
  • 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 the effect of illness.
  • Eg:  environment (water, Soil,  plant),  human and animal
  • Carriers:  an individual capable of transmitting pathogen/  agent without display symptoms is referred to as a carrier:
    1. Passive carriers: contaminated with an agent & can mechanically transmit it to another post (the passive carrier is not infected)
    2. Active carriers: an infected individual who can transmit the disease to others
      • Incubatory: who can transmit the agent during the incubation before clinical illness begins
      • Convalescent:  recovered from their illness but remain capable of transmitting to others.
      • Chronic: who continue to herbers agent/pathogen surcharge hepatitis B &Salmonella typhi.

Mode of transmission of Disease

Mode of transmission is the manner is which an agent is transmitted from its reservoir to a susceptible host.

Mode of Transmission of Disease

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 & the infectious agent undergoes replication for development for both in vector.

 Transmission  less than 1 metre (Direct contact)  more than 1 metre (Indirect)

 Zoonosis:  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, raccoons), HIV, Ebola, SARS

Disease transmitted by Arthropods as vectors

Disease transmitted by Arthropods as vectors-The Pharmapedia
Disease transmitted by Arthropods as vectors
Mosquitoes: Life Cycle: 4 distinct stage
1.Egg (Require blood meal to develop)
4.Adult (controlled by DDT/Pyrethrum
CulexFilariasis, Japanese encephalitis, *West nile virus (*viral disease)
Aedes (only bite during day times)*Yellow fever, *Chickengunya, *Zika virus, *Dengue, Rift vally disease  (*viral disease)
Mosquito repellents: Diethyltolumide

Arboviral disease (arthropod-borne viral disease)

Indirect transmission refers to the transfer of an infectious agent from a reservoir to a host by suspended air particles, inanimate objects (vehicles), or animate intermediaries (vectors).

Airborne transmission occurs when infectious agents are carried by dust or droplet nuclei suspended in air. Airborne dust includes material that has settled on surfaces and become resuspended by air currents as well as infectious particles blown from the soil by the wind. Droplet nuclei are dried residue of less than 5 microns in size. In contrast to droplets that fall to the ground within a few feet, droplet nuclei may remain suspended in the air for long periods of time and maybe blown over great distances. Measles, for example, has occurred in children who came into a physician’s office after a child with measles had left, because the measles virus remained suspended in the air.(46)

Vehicles that may indirectly transmit an infectious agent include food, water, biologic products (blood), and fomites (inanimate objects such as handkerchiefs, bedding, or surgical scalpels). A vehicle may passively carry a pathogen — as food or water may carry the hepatitis A virus. Alternatively, the vehicle may provide an environment in which the agent grows, multiplies, or produces toxin — as improperly canned foods provide an environment that supports the production of botulinum toxin by Clostridium botulinum.

Vectors such as mosquitoes, fleas, and ticks may carry an infectious agent through purely mechanical means or may support growth or changes in the agent. Examples of mechanical transmission are flies carrying Shigella on their appendages and fleas carrying Yersinia pestis, the causative agent of plague, in their gut. In contrast, in biologic transmission, the causative agent of malaria or guinea worm disease undergoes maturation in an intermediate host before it can be transmitted to humans.

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