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TOPIC 5.docx

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University of Waterloo
HLTH 101
Glenn Ward

TOPIC 5: CHANGES IN HEALTH THROUGH EMERGING AND RE-EMERGING INFECTIOUS DISEASES 1: THE BIOLOGY OF INFECTIOUS DISEASES 1. INFECTIOUS ORGANISMS Infectious organisms can be classified in a number of ways. In this course, I have chosen to classify them according to their size first, and then according to their taxonomic classification. i. Subcellular Organisms  in fact, these are not technically organisms but are complex microscopic agents that usually (but not always) reside within living cells)  we’ll discuss two main types: viruses and prions VIRUSES  exist basically as RNA or DNA molecules (generally not both) encased inside a coat of protein*  about 1500 viruses have been identified (so far), only a small number of which are believed to affect human health  viruses have a simple existence outside host cells* o they are obligate intracellular parasites, which means that they must enter the host cell to reproduce  attach to cell membrane  enter cell  release coat  replicate nucleic acids  reconstitute protein coat  progeny exit cell o all cells are susceptible to viral infection  even single celled organisms such as bacteria can be infected with viruses (bacteriophages are viruses that infect bacteria)  viral reproduction can damage the host cell* o when the virus reproduces, the offspring viruses can then invade a new cell and replicate, resulting in damage to the new cell, etc.  destroy host cell  alter host cell function  elicit host immune response  destroy host cell o does not require rapid reproduction  because of the ability of the virus to travel through the body, there is a low association between the route of entry and the tissue most affected by the virus  common diseases caused by viruses include: i. common colds ii. HIV (AIDS) iii. influenza iv. HPV (human papilloma virus) v. herpes vi. measles PRIONS  prions are characterized as proteinaceous infectious particles  they lack DNA and RNA and consist mainly of protein-like molecules  they appear to exist in all brains o their mechanism of action is still not well-understood, but research suggests that prions are susceptible to alterations in their shape or configuration  if their configuration is altered, these altered prions can reproduce and propagate throughout nearby cells  this eventually leads to the death of neural cells in which the prions reside  altered prions can be transmitted through food and surgical instruments as infectious agents  rare diseases caused by prions: i. transmissible spongiform encephalopathy ii. CJD (Cruetzfeldt-Jakob disease) iii. vCJD (variant CJD) (believed to be a subtype of CJD possibly associated with bovine spongiform encephalopathy [BSE] ii. Unicellular Organisms  unicellular organisms are those that exist as single cells  as such, they are more complex that viruses and prions, but are still relatively simple  however, they are highly specialized for life in their respective environments  the two types of cellular organisms associated with infectious disease are bacteria and protozoa BACTERIA  bacteria are unicellular prokaryotic organisms  structurally, they are highly adapted for survival* 1) most (but not all) bacteria are enclosed in a capsule o among other things, this capsule makes the bacteria resistant to phagocytosis o the capsule also plays a role sometimes in the virulence of the bacteria 2) all bacterial cell walls contain peptidoglycan (note that some sources will refer the Archaebacteria as an exception to this rule. While the cell walls of Archaea do not contain peptidoglycan, it has become common over the past decade to refer to them as a prokaryote domain separate from bacteria. As they are not known at this time to contribute to human health problems, they will not be included in this discussion) o this substance provides rigidity and resistance to lysis o it also determines the gram positive and gram negative status of the bacterium (important for determining appropriate antibiotic treatment) 3) some bacteria contain cytoplasmic organelles called plasmids o these organelles consist of self-replicating, non-chromosomal DNA which can be transferred to other bacteria o in this way. bacteria can sometimes transfer virulence and genetic resistance to other bacterial cells 4) some bacteria also produce spores o these spores contain a condensed version of the bacterial cell, including the cell’s DNA, within a specialized coat that can resist heat, humidity, harmful chemicals, etc. to ensure the survival and generation of the bacteria when it is not inhabiting a living host  bacteria are characterized by their rapid rate of reproduction (by binary fission)* o when they are associated with illness, they can often produce illness within hours of infection of the host o rapid mutation rate o selection of the fittest  most bacteria are nonpathogenic and essential for human life  when they are harmful, their actions are generally due to:* 1) toxins produced by reproducing bacteria: o that are released in the human body after infections o that are released in food or water consumed by humans 2) tissue invasion of the bacteria leading to inflammation and/or death of the tissue o elicit immune response  the effects of bacteria are usually associated with the site of entry  common diseases caused by bacteria include: i. cholera ii. syphilis, gonorrhea and chlamydia iii. botulism iv. E. Coli infection v. Lyme disease vi. gastric ulcers caused by H. pylori vi. food poisoning PROTOZOA  protozoa are unicellular eukaryotic organisms  they are ubiquitous o more than 20,000 species are known to exist (only a few are harmful to humans)  protozoa have a complicated life cycle: o they have more than one host o they also have the ability to live outside the host o this ability is due to a process called encystation  this process involves the formation of a cyst enclosed in a protective capsule*  when the protozoan is in
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