Pathogens: Definition and Meaning of Pathogens, Pathogenicity (For CBSE, ICSE, IAS, NET, NRA 2022)

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Meaning of Pathogens

  • Pathogens can be defined as the infectious agents that can cause illness and infection.
  • The term pathogen came into use in the 1880s.
  • Infectious diseases are caused by various pathogenic microorganisms for e. g. virus, bacteria, protozoan, fungi, and other parasites.
  • The infectious diseases can be transmitted by animals, humans, insects, or other agents.


  • It is the potential disease-causing capacity of pathogens.
  • An organism can be pathogenic or non-pathogenic, but one cannot be more pathogenic than the other.
  • A pathogen may be described in terms of its ability:
    • To produce toxins.
    • Enter tissue
    • Hijack nutrients
    • To immunosuppress the host
    • Immunosuppress is the reduction of activation or efficacy of the immune system.

Context-Dependent Pathogenicity

  • Pathogenicity depends on the microbial ecosystem as per the modern view.
  • A bacterium may participate in opportunistic infections in immunocompromised hosts. Opportunistic infection means infections caused by pathogens taking advantage of an opportunity normally not available.
  • Immunodeficiency or immunocompromise can be defined as a state in which the immune system՚s ability to fight infectious disease and cancer is compromised or entirely absent.

Classification of Pathogens

Class I

  • They should not be part of an identified group.
  • The non-pathogenicity of the species has been demonstrated authoritatively both in vivo and in vitro testing.

Class II

  • Species spreading diseases in humans or animals might not spread to the humans.
  • Having the essential prophylaxis or therapy.

Class III

  • Species that cause serious disease in humans, spread to the humans.
  • Having the essential prophylaxis or therapy.

Class IV

  • Species causing serious human disease likely to disseminate in the human population.
  • No prophylaxis or therapy exists.

Types of Pathogens


  • Algae are single-celled eukaryotes.
  • Eukaryotes are those organisms with cells having a nucleus enclosed within a nuclear envelope.
    • Non-pathogenic although pathogenic varieties do exist.
  • Protothecosis
    • A disease found in dogs, cats, cattle, and humans.
    • This disease is caused by a type of green alga known as prototheca that lacks chlorophyll.


  • Bacteria are microorganisms made of a single cell.
  • Pathogenic bacteria can cause infectious diseases.
  • They are diverse having a variety of shapes and features.
Commensals vs Pathogens Mechanism


  • They can be found indoors, outdoors, and on human skin.
  • The typical fungal spore size is < 4.7 μm in length.
  • Fungi cells contain a nucleus and other components protected by a membrane.


  • These are the organisms that behave like tiny animals, living in or on a host.
  • Parasitic infections are more common in tropical and subtropical regions.

Three Main Types of Parasites


Commonly known as worms are larger, multi-celled organisms that can live inside or outside the human body.

  • These are single – celled organisms that that can live and multiply in the body.
  • Ectoparasites are multi-celled organisms that live on or feed off the skin.
  • This also includes some insects like ticks and mosquitos.


  • These are the smallest pathogens known.
  • These are composed solely of a short strand of circular, single-stranded RNA.


  • These are misfolded proteins that can transfer their misfolded state to other normally folded proteins of the same type.
  • They do not contain any DNA or RNA and cannot replicate.


  • These are small particles typically between 20 and 300 nanometers in length.
  • They contain RNA or DNA.
  • They require a host cell to replicate.

Pathogen Hosts

Pathogen Hosts


Q. What are pathogens?


A pathogen can be referred to as an infectious agent or simply a germ that can produce diseases. Pathogenicity is the potential disease-causing capacity of pathogens.

Q. What are the four most common types of pathogens?


The four most common types of pathogens are:

i) Viruses

ii) Bacteria

iii) Fungi

iv) Parasites

Q. In how many classes pathogens can be classified?


Pathogens can be classified into four classes namely Class I, Class II, Class III and Class IV.

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