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1: Introduction to Infectious Diseases

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    Examples of reading assignment:

    Introduction to infectious diseases vocabulary


    any abnormal bodily condition that impairs functioning and can usually be recognized by signs (what can be seen/felt e.g., redness, swelling) and symptoms (what patients “feels” e.g. nausea, malaise, pain).


    Figure 1.1: Child infected with smallpox/variola virus. (Public Domain; CDC/James Hicks)

    Infectious disease

    disease caused by colonization of host organism by pathogenic microbes

    • -pathos=suffering genic=to make
    • -pathogenic microbes e.g. prions, viruses, bacteria, protozoa

    EID=Emerging Infectious Diseases:

    \described as infectious diseases appearing /”emerging” within the last 20-30 years. e.g. zoonotic avian influenza, Ebola


    Figure 1.2: Influenza virus model

    • Included as EID’s are “re-emerging infectious diseases”, diseases which had decreased in prevalence at one time in history and now are once more spreading through human populations.
    • The Institute of Medicine (IOM) also includes the recently evolved antibiotic-resistant pathogens as causes of EID’s. -e.g. MRSA/ Methicillin Resistant Staphylococcus aureus, MDR-TB/Multi-Drug Resistant Tuberculosis.


    An organism/agent (e.g. virus, bacterium, helminth) which lives at the expense of another (the host). The terms parasite and pathogen are sometimes used interchangeably.


    from Greek patho=suffering, An agent/organism which causes disease/harm to another organism (the host). pathogenic: capable of causing disease e.g. pathogenic microbe compared to non-pathogenic microbe


    Greek oikos=house -logy= study of : the study of interactions between organisms and their environment

    Infectious Disease Medical ecology

    the study of the interactions between host, pathogens and environment leading to infectious disease.


    The living together of two (or more) dissimilar organisms. 3 types of symbiotic relationships:

    1. mutualism: both symbiotic partners benefit (e.g. non-pathogenic E. coli living in your intestinal tract)
    2. commensalism: one partner benefits, the other neither benefits nor is harmed, e.g. Staphylococcus aureus living harmlessly in your nose)
    3. parasitism: one partner (the parasite/pathogen) benefits at the cost of th other partner (the host is harmed). e.g. Staphylococcus aureus causing an infection of a cut on your hand; E. coli causing a bladder infection

    -Note: as indicated with Staphylococcus aureus and E.coli, symbiotic relationships can change!

    Opportunistic pathogen

    A microbe which normally does not cause disease but given special circumstances, may cause disease. e.g. E.coli causing bladder infection / cystitis or Pneumocystis jirovecii causing pneumonia in someone with HIV/AIDS

    Microbial pathogens

    Pathogens/parasites too small to be seen with the “naked eye” / not using magnifying instrument, e.g. prions, viruses, bacteria, protozoa.

    Macroscopic parasites

    parasites which can be seen with the “naked eye”/without magnification. The general categories:

    1. ectoparasites: parasites which live/feed on the outside (“ecto-“) of the body. Many are blood-feeders, e.g. insects including fleas, mosquitoes, lice; arachnids including ticks.
    2. endoparasites: parasites which live within (“endo-“) the host.e.g. “worms” or helminths such as Ascaris lumbricoides

    Arthropod vectors

    Many of the blood-feeding ectoparasites act as arthropod vectors, organisms which transfer microbial pathogens between hosts.

    • -eg. Mosquitoes are vectors of West Nile virus and Plasmodium/malaria.
    • *arthropod= “jointed foot”; invertebrates (lack backbones) with segmented bodies and jointed limbs; have “exoskeletons:” -e.g. insects, arachnids

    Figure 1.3: A mosquito as a vector.

    Agents of infectious diseases

    Prions Viruses Prokaryotes Eukaryotes Fungi:
    spongiform encephalopathies HIV Bacteria Protozoa Candida/yeast
    “Mad Cow Disease” Ebola Mycobacterium tuberculosis/TB Plasmodium/malaria Coccidioides
    Kuru Hanta Staphylococcus Cryptosporidium “Macro” parasites:
    Creutzfeldt-Jakobs Disease Polio Bacillus anthracis/anthrax Giardia elminths/”worms”
    Chronic Wasting Disease Rabies E. coli Trypanosoma/sleeping sickness Ectoparasites/vectors:
      Hepatitis Salmonella   mosquitoes, ticks, fleas
      Smallpox Chlamydia/STD    
        Treponema pallidum/syphilis    


    Acellular agents Cellular organisms

    Determinants of Infectious Disease

    To cause an infectious disease, a pathogen must be able to:

    1. Initially be transported to host: food, water, sex, arthropod vectors, aerosols, animal bites, blood, congenital
    2. Adhere to, colonize or invade the host
    3. Multiply (grow) or complete its life cycle on or in the host
    4. Initially evade host defense mechanisms
    5. Possess the mechanical, chemical or molecular ability to damage the host.
    6. Leave host, invade new host


    Discussion questions and study guide questions for quizzes and exams.

    Prepare for lecture 2, TURN-IN your answers at the start of lecture 2, make sure your name is on your homework sheet. + 5 points

    People will be asked to answer these questions during our class discussion in lecture 2.

    1. What are “infectious diseases”? Provide 1 example
    2. What are EID’s, Emerging Infectious Diseases? Provide 2 examples
    3. 3. Medical ecology explores how interactions between pathogens, hosts and the environment lead to infectious diseases. Draw and label a medical ecology Venn Diagram indicating pathogen, host, environment and infectious disease.
    4. 4. Describe pathogen, host and environment factors which could influence prevalence of infectious diseases by filling in the table below:

    Host factors

    Pathogen factors

    Environmental factors


    5. Define/describe the following terms and provide a specific example of each.

    1. symbiosis
    2. parasitism
    3. parasite
    4. host
    5. pathogen

    6. Do an online computer search for “the political ecology of infectious disease”.

    What is your definition of the political ecology of infectious diseases? Provide a specific example.

    1: Introduction to Infectious Diseases is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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