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Thread: Some Terminology in Infectious Diseases

  1. #1

    Default Some Terminology in Infectious Diseases

    Host: Any organism capable of supporting the nutritional and physical growth requirements of another is called a host.

    Infection: The term infection describes the presence and multiplication within a host of another living organism, with subsequent injury to the host.

    Colonization: Colonization describes the act of establishing a presence, a step required in the multifaceted process of infection.

    Commensalism: Although the colonizing bacteria acquire nutritional needs and shelter, the host is not adversely affected by the relationship. This interaction is called commensalism.

    Mutualism: Mutualism is applied to an interaction in which the microorganism and the host both derive benefits from the interaction.

    Parasitic relationship: Parasitic relationship is one in which only the infecting organism benefits from the relationship and the host either gains nothing from the relationship or sustains injury from the interaction.

    Infectious diseases: If the host sustains injury or pathologic damage in response to a parasitic infection, the process is called an infectious disease.
    Last edited by CheneyHsiung; Wed 16th July '14 at 8:39pm.
    Clinical Pharmacy Specialist - Hematology

  2. #2
    PharmD Year 1 TomHsiung's Avatar
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    Chengdu, Sichuan, China

    Default Characteristics of Agents of Infectious Diseases

    The agents of infectious diseases include prions, viruses, bacteria, Richettsiaceae & Chlamydiaceae, fungi, and parasites, six categories in total.


    Prions are a kind of infectious agents with no nucleic acid but can survive, reproduce, infect, and be transmitted. The etiology of prions is a kind of protein called scrapie prion proteins (PrPsc), which are actually altered or mutated forms of a normal host protein called cellular PrPc. The PrPsc is very stable and, therefore, is resistant to many antibiotics (in fact, prions lack reproductive and metabolic functions, so the currently available antimicrobial agents are useless against them). Based on current models, it is believed that PrPsc binds to the normal PrPc on the cell surface, causing it to be processed into PrPsc, which is released from the cell and then replenishes the PrPc and the cycle continues. As PrPsc accumulates, it spreads within the axons of the nerve cells, causing progressively greater damage of host neurons and the eventual inacpacitation of the host.


    Viruses are the smallest obligate intracellular pathogens. They have no no organized cellular structures but instead consist of a protein coat, or capsid, surrounding a nucleic acid core, or genome, of RNA or DNA - never both. Some viruses are enclosed within a lipoprotein envelop derived from the cytoplasmic membrane of the parasitized host cell. Of note, viruses are incapable of replication outside of a living cell. They must penetrate a susceptible living cell and use the biosynthetic structure of the cell to produce viral progeny. Not every viral agent causes lysis and death of the host cell during the course of replication. Some viruses enter the host cell and insert their genome into the host cell chromosome, where it remains in a latent, nonreplicating state for long periods without causing disease. Under appropriate stimulation, the virus undergoes active replication and produces symptoms of disease months to years later.

    In addition to causing infectious diseases, certain viruses also have the ability to transform normal host cells into malignant cells during the replication cycle.


    Bacteria lack an organized nucleus but they can autonomously replicate. Bacteria contain both DNA and RNA. They contain no organized intracellular organelles, and the genome consists of only a single chromosome of DNA. Of note that many bacteria transiently harbor smaller extrachromosomal pieces of circular DNA called plasmids. Occasionally, plasmids contain genetic information that increases the virulence or antibiotic resistance of the organism.

    Bacteria are surrounded by a flexible lipid membrane, called the cytoplasmic membrane. And outside the membrane it is enclosed within a rigid cell wall. Most bacteria produce a cell wall composed of a distinctive polymer known as peptidoglycan. This polymer is produced only by prokaryotes and is therefore an attractive target for antibacterial therapy.

    Certain bacteria are motile as the result of external whiplike appendages called flagella. Bacteria can also produce hairlike structures projecting from the cell surface called pili or fimbriae, which enable the organism to adhere to surfaces such as mucous membranes or other bacteria.

    Most prokaryotes reproduce asexually by simple cellular division. The growth rate of bacteria varies significantly among different species and depends greatly on physical growth conditions and the availability of nutrients. In nature, however, bacteria rarely exist as single cells floating in an aqueous environment. Rather, bacteria prefer to stick to and colonize environmental surfaces, producing structured communities called biofilms.

    Two subtypes belong to bacteria are spirochetes and mycoplasmas.

    The spirochetes are an eccentric category of bacteria that are mentioned separately because of their unusual cellular morphology and distinctive mechanism of motility. Technically, the spirochetes are gram-negative rods but are unique in that the cell's shape is helical and the length of the organism is many times its width. A series of filaments are wound about the cell wall and extend the entire length of the cell. These filaments propel the organism through an aqueous environment in a corkscrew motion.

    Spirochetes are anaerobic organisms and comprise three genera: Leptospira, Borrelia, and Treponema.

    Mycoplasmas are unicellular prokaryotes capable of independent replication. These organisms are less than one third the size of bacteria at approximately 0.3 um at their largest diameter and contain a small DNA genome approximately one half the size of the bacterial chromosome. The cell is composed of cytoplasm surrounded by a membrane but, unlike bacteria, the mycoplasmas do not produce a rigid peptidoglycan cell wall. The mycoplasmas affecting humans are divided into three genera: Mycoplasma, Ureaplasma, and Acholeplasma.

    Rickettsiaceae, Anaplasmataceae, Chlamydiaceae, and Coxiella

    All of these four are obligate intracellular pathogens, like the viruses, but produce a rigid peptidoglycan cell wall, reproduce asexually by cellular division, and contain RNA and DNA, similar to the bacteria.

    The Rickettsiaceae depend on the host cell for essential vitamins and nutrients. The Rickettsiaceae infect but do not produce disease in the cells of certain arthropods (colonization and/or parasitic relationship). The organisms are accidentally transmitted to humans through the bite of the arthropod.

    The Chlamydiaceae are slightly smaller than the Rickettsiaceae but are structurally similar and are transmitted directly between susceptible vertebrates without an intermediate arthropod host. Transmission and replication of Chlamydiaceae occur through a defined life cycle. The infectious form, called an elementary body, attaches to and enters the host cell, where it transforms into larger reticulate body. This undergoes active replication into multiple elementary bodies, which are then shed into the extracellular environment to initiate another infectious cycle.

    The Anaplasmataceae (including Ehrlichia, Anaplasma, Neorickettsia, and Wolbachia) are also obligate intracellular organisms, some of which have a tick vector. These organisms target host mononuclear and polymorphonuclear white blood cells for infection and, similar to the Chlamydiaceae, multiply in the cyytoplasm of infected leukocytes within vacuoles called morulae. However, different from Chlamydiaceae, Anaplasmataceae do not have a defined life cycle and are independent of the host cell for energy production.

    The Coxiella is a gram-negative intracellular organism that infects a variety of animals, including cattle, sheep, and goats. In humans, Coxiella infection produces a disease called Q fever, characterized by a nonspecific febrile illness. The organism produces a highly resistant sporelike stage that is transmitted to humans when contaminated animal tissue is aerosolized or by ingestion of contaminated milk.


    The fungi are free-living, eukaryotic saprophytes found in every habitat on earth. Some are members of the normal human microflora. Fortunately, few fungi are capable of causing diseases in humans, and most of these are incidental, self-limited infections of skin and subcutaneous tissue. Serious fungal infections are rare and usually initiated through puncture wounds or inhalation.

    The fungi can be separated into two groups, yeasts and molds, based on rudimentary differences in their morphology. Yeasts are single-celled organisms, approximately the size of red blood cells, which reproduce by a budding process. A limited number of fungi are capable of growing as yeasts at one temperature and as molds at another. Reproduction may be sexual or asexual.

    Yeast and molds produce a rigid cell wall layer that is chemically unrelated to the peptidoglycan of bacteria and is therefore not susceptible to the effects of penicillin-like antibiotics.


    Parasites include protozoa, helminths, and arthropods.

    The protozoa are unicellular animals with a complete complement of eukaryotic cellular machinery, including a well-defined nucleus and organelles. Reproduction may be sexual or asexual, and life cycles may be simple or complicated, with several maturation stages requiring more than one host for completion. Most are saprophytes, but a few have adapted to the accommodations of the human environment and produce a variety of diseases.

    Protozoan infections can be passed directly from host to host such as sexual contact, indirectly from host to host such as through contaminated water or food, or by way of an arthropod vector.

    The helminths are a collection of wormlike parasites that include the nematodes or roundworms, cestodes or tapeworms, and trematodes or flukes. The helminths reproduce sexually within the definitive host, and some require an intermediate host for the development and maturation of offspring. Human can serve as the definitive or intermediate hsot and, in certain diseases such as trichinosis, as both. Transmission of helminth diseases occurs primarily through the ingestion of fertilized eggs (ova) or the penetration of infectious larval stages through the skin - directly or with the aid of an arthropod vector.

    The parasitic arthropods of humans and animals include the vectors of infectious diseases (e.g., ticks, mosquitoes, biting flies) and the ectoparasites. The ectoparasites infest external body surfaces and cause localized tissue damage or inflammation secondary to the bite or burrowing action of the arthropod. Transmission of ectoparasites occurs directly by contact with immature or mature forms of the arthropod or its eggs found on the infested host or the host's clothing, bedding, or grooming articles such as combs and brushes. Many of the ectoparasites are vectors of other infectious diseases, including endemic typhus and bubonic plague (fleas) and epidemic typhus ((lice).

    (The End)
    Last edited by admin; Sat 13th September '14 at 4:54pm.
    B.S. Pharm, West China School of Pharmacy, Class of 2007, Health System Pharmacist, RPh. Hematology, Infectious Disease.

  3. #3
    PharmD Year 1 TomHsiung's Avatar
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    Default Portal of Entry of Infectious Agents

    Generally there are four ways for infectious agents to enter the human body.


    2.Direct Contact




    The portal of entry dose not dictate the site of infection (e.g., ingested pathogens may penetrate the intestinal mucosa, disseminate through the circulatory system, and cause diseases in other organs such as the lung or liver). Whatever the mechanisms of entry, the transmission of infectious agents is directly related to the number of infectious agents absorbed by the host.

    For the virulence please refer to
    Last edited by TomHsiung; Sat 10th January '15 at 6:39pm.
    B.S. Pharm, West China School of Pharmacy, Class of 2007, Health System Pharmacist, RPh. Hematology, Infectious Disease.

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    PharmD Year 1 TomHsiung's Avatar
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    Jan 2013
    Chengdu, Sichuan, China
    B.S. Pharm, West China School of Pharmacy, Class of 2007, Health System Pharmacist, RPh. Hematology, Infectious Disease.

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