Parasitic Infections: Source of Infection, Mode of Transmission, and Prevention

By Nisha Rijal •  Updated: 04/11/22 •  5 min read

A parasite is a living organism (maybe a bug, bacteria, fungi, protozoa) that lives upon and derives nutrients directly from a host (another living organism), without giving any benefit to the host. In absence of a host, a parasite cannot live, grow and multiply, therefore a parasite normally doesn’t kill the host but it can spread diseases, and some of these can be fatal. Parasites can be unicellular or multicellular and are often smaller as compared to the host.

Hookworm, bed bugs, lice, honey fungus, Entamoeba, etc are a few examples of parasites

Types of parasites

On the basis of their location, parasites may be classified as:

  1. Ectoparasite: Organisms that inhabit the surface of the body of the host without penetrating into the tissues are called ectoparasites. For example, human lice, fleas, ticks, mites, etc. They are important vectors transmitting the pathogenic microbes. The infection by these parasites is called an infestation.
  2. Endoparasite: Organisms that live within the body of the host (in the blood, tissues, body cavities, and other organs) are called endoparasites. Invasion by the endoparasite is called an infection. In humans, all pathogenic protozoan and helminthic parasites are endoparasites. For examples roundworm, hookworm, amoeba, etc.
Parasites Infecting Humans
Various Ectoparasites and Endoparasites of Humans

Endoparasites can be further subdivided into the following types based on their relation to the host

  1. Obligate parasites: Parasites that complete a phase of their lifecycle in the host, therefore, without a host they cannot complete their life cycle.  For example Plasmodium (malarial parasite), Toxoplasma gondii etc
  2. Facultative parasites: Parasites that under favorable conditions may live either a parasitic life or free-living life. Examples include Acanthamoeba, Naegleria fowleri etc
  3. Accidental parasites: Parasites that infect an unusual host. For example, Echinococcus granulosus is common in dogs but may infect humans accidentally.
  4. Aberrant parasites or wandering parasites: Parasites that infect a host where they cannot live or develop further (e.g., Toxocara in humans).

Source of infection of parasites

Humans may acquire parasites from various sources such as contaminated soil and water, food, and contact with

  1. Contaminated soil and water: Soil polluted with human excreta may contain eggs of the parasites (such as hookworm, Ascaris species, Strongyloides species, and Trichuris species) and is an important source of infection. Similarly, infection may occur by drinking water contaminated with human excreta containing cysts of E. histolytica or Giardia lamblia.
  2. Raw or undercooked meat: Raw beef containing the larvae of Cysticercus bovis and pork containing Cysticercus cellulosae are some examples where undercooked meat acts as a source of infection.
  3. Other sources of infection:
    1. Aquatic source: Freshwater fishes (source of Diphyllobrothium latum), crab or crayfishes (source of Paragonimus westermanii), aquatic plants such as watercress (source of Fasciola hepatica)
    2. Bloodsucking insects: Mosquitoes (source of Plasmodium, Wuchereria bancrofti), tsetse fly (source of Trypanosoma spp), sandflies (Leishmania spp)
    3. Domestic animals: Animals such as cats (source of T.gondii) and Dog (source of E.granulosus), could also be the source of infection of parasites
    4. Man: The human itself is the host to several parasites and can excrete infective stages (eggs and larvae) and become the source of parasites such as E.histolytica, G.lamblia, E.vermucularis, etc

Modes of transmission

The infective stages of various parasites may be transmitted from one host to another in the following ways:

  1. Oral or feco-oral route: It is the most common mode of transmission of the parasites. Infection is transmitted orally by ingestion of food, water, or vegetables contaminated with feces containing the infective stages of the parasite, soiled fingers, etc (e.g., cysts of E. histolytica, and ova of Ascaris lumbricoides)
  2. Penetration of the skin and mucous membranes: Infection is transmitted by the penetration of the larval forms of the parasite through unbroken skin (e.g., filariform larva of Strongyloides stercoralis and hookworm can penetrate through the skin of an individual walking bare-footed over fecally contaminated soil), or by the introduction of the parasites through insect bite of bloodsucking insect vectors (e.g., Plasmodium species, Leishmania species, and Wuchereria bancrofti)
  3. Physical and sexual contact: Parasites such as E.gingivalis is transmitted from person to person by kissing or drinking from contaminated drinking utensils. Parasite such as Trichomonas vaginalis is transmitted by sexual contact. Others such as Entamoeba, Giardia, and Enterobius are also transmitted rarely by sexual contact among homosexuals.
  4. Vertical transmission: Infections with Toxoplasma gondii, Plasmodium spp., and Trypanosoma cruzi may be transmitted transplacentally from an infected mother to a fetus.
  5. Blood transfusion: Certain parasites like Plasmodium species, Babesia species, Toxoplasma species, Leishmania species, and Trypanosoma species can be transmitted through transfusion of blood or blood products.
  6. Autoinfection: Few intestinal parasites may be transmitted to the same person by contaminated hand (external autoinfection) or by reverse peristalsis (internal autoinfection). It is observed in Cryptosporidium parvum, Taenia solium, Enterobius vermicularis, Strongyloides stercoralis, and Hymenolepis nana.

Prevention of parasitic infections


Nisha Rijal

I am working as Microbiologist in National Public Health Laboratory (NPHL), government national reference laboratory under the Department of health services (DoHS), Nepal. Key areas of my work lies in Bacteriology, especially in Antimicrobial resistance.

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12 responses to “Extracellular and Intracellular Bacteria”

  1. Meg Mangin says:

    Good info. Thanks!

  2. Renjan says:

    I have been working on a bacteria and I found it to be intracellular . TEM micrographs suggests that these bacteria forms vacuoles and tend to multiply within the vacuolated cups when infected with RAW macrophages cells.

    My query: Can anyone suggest a paper wher they prove a particular bacteria to be intracellular. want a ref paper.TQ

  3. Anonymous says:

    Why not all bacteria ?

  4. Anonymous says:

    Thank you

  5. Manish says:

    Why the immune response to intracellular pathogens is different from extracellular pathogens?

    • Tankeshwar Acharya says:

      Dear Manish
      Thank you for your query. Did you studied Immunology? Your question requires understanding of various immunological concepts. If you have please look the chapters of Antigen processing and presentation. You will get answer there.

  6. Anonymous says:

    Tankeshwar Acharya, thank you for your blog explaining intracellular and extracellular bacteria. I am doing some desktop research on chronic urinary tract infections and found your description helpful. Did you know there are recent findings that incriminate intracellular bacteria and/or biofilm bacterial infections in chronic lower urinary tract symptoms (LUTS). Scientists in the US (Wolfe, A & Braubaker, L) and UK (Malone-Lee, J & Rohn, J) have recently discovered through the use of molecular technology, that urine in not at all sterile and up to 450 different bacterial species are found living in the bladder of healthy people. They believe these bacterial communities could be working in the same way that gut bacteria do by protecting the bladder from invading pathogenic bacteria. But in some people, this protective mechanism stops working for some reason, and a pathogenic bacteria takes over and invades cells or forms biofilm on the surface of cells (Hultgren, S). Being a microbiologist, I thought you’d be interested in this topic.

    Renjan, this paper talks about intracellular bacterial communities:

    • Tankeshwar Acharya says:

      Dear Friend thank you so much for your comment and this useful information, I will go through this article. Generally we regard Blood, Urine and other body fluids are sterile; after availability of molecular techniques and newer diagnostics, scientist/researcher are claiming that Blood is not sterile either. For routine diagnostics; so far we believe and practice that these specimen are sterile or organisms if present in these specimen do not grow on our routine culture media. I am hopeful that with the use of novel diagnostic techniques, researcher/scientist will discover and validate new facts/findings.

  7. Anonymous says:

    The above link was about intracellular bacterial communities, which I shared because someone above asked for a paper that had evidence of bacteria being intracellular.

    In relation to my comments above about urine not being sterile, you might be interested in reading the Wolfe & Baubaker paper. It’s hot science. I agree, I hope new technology and techniques provide medical solutions to a lot of unanswered questions. I hope you find it interesting.

  8. Mr Acharya
    I am biomedical science student
    how does immune system switch between cell mediated and humeral in relation to intra or extra cellular bacteria..or both work along

  9. Anan says:

    Thank you prof for you great explanitation, but I have a query, does treponema as a jenus considered one of the obligate intracellular bacteria? I really need your help

  10. Musa, B. says:

    Please what are the factors responsible for the predilection of intra-cellular bacteria? Thank you

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