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Fall 2010: Topic 3 Lecture Notes this is the information is topic 3 called "ecology of the human body and health:infectious diseases" including infectious organisms, the biology of infectious diseases, the epidemiology of infectious diseases, Internal Eco


Department
Health Studies
Course Code
HLTH 101
Professor
Glenn Ward

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Tuesday September 28, 2010
Topic three: The Ecology of The Human Body and Health: Infectious Diseases
1. Infectious Organisms
Infectious organisms can be classified in a number of ways; first by size,
then according to their taxonomic classifications
i. SUBCELLULAR ORGANISMS
- Not technically organism, but complex microscopic agents that
usually reside within living cells
- Two types: viruses and prions
a. Viruses
Exists as RNA or DNA molecules encased in a protein coat
1500 viruses have been identified, only a small number
affect human health
Have simple existence outside host cell
Obligate intracellular parasites: must enter host cell
to reproduce
All cells are susceptible for viral infection (even
bacteria: bacteriophages)
Viral reproduction can damage host cell (offspring invades
new cells and replicates, damaging new cell, etc.)
Viruses travel throughout body, therefore low association
between entry route and tissue affected
Diseases caused include common cold, HIV/AIDS,
influenza, HPV, herpes, measles
b. PRIONS
Proteinacaous infectious particles
Lack DNA and RNA: mainly protein like molecules
Exist in all brains
Susceptible to alterations in shape
If altered, altered prions reproduce and spread to
nearby cells
Leads to death or neural cells with prions
Altered prions can be transmitted through food/surgical
instruments as infectious agents
Diseased caused include transmissible spongiform
encephalopathy, CJD (Creutzfeldt-Jakob disease)
vCJD (form of CJD associated with bovine spongiform
encephalopathy (BSE))
ii. UNICELLULAR ORGANISMS
- Exist as single cells
- More complex than viruses and prions (still simple)
- Highly specialized for respective environments

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- Two types associated with infectious disease: bacteria and
protozoa
a. Bacteria
unicellular prokaryotic organisms
structurally highly adapted for survival
1. most enclosed in a capsule
capsule allows resistance to phagocytosis
capsule plays role in virulence of bacteria
2. Bacterial cell walls contain peptidoglycan (except
archaebacteria)
Provides rigidity and resistance to lysis
Determines gram positive and gram negative
status of bacterium (for determining antibiotic
treatment)
3. Some bacteria have cytoplasmic organelles called
plasmids
Plasmids have self-replicating , non chromosomal
DNA which can be transferred to other bacteria
Bacteria sometimes transfer virulence and genetic
resistance to other bacterial cells
4. Some bacteria produce spores
Spores have condensed version of bacterial cell
(with DNA) within a specialized coat to resist heat,
humidity, harmful chemicals, etc. To ensure survival
and generation when not inhabiting a living host
Bacteria are characterized by rapid rate of reproduction
(binary fission). When associated with illness, can produce
illness within hours of infecting host
Most are non-pathogenic and essential for human life
When harmful, actions are due to
1. Toxins produced by bacterial reproduction
Released in human body after infections
Released in food or water consumed by humans
2. Tissue invasion of bacteria leading to
inflammation/tissue death
Effects of bacteria usually associated with site of entry
Diseases include cholera, syphilis, gonorrhoea, Chlamydia,
botulism, E. Coli, Lyme disease, gastric ulcers (H. Pylori), and
food poisoning
b. Protozoa
Unicellular eukaryotic organisms
Ubiquitous (more than 20 000 species, only a few are
harmful to humans)
More complication life cycle
More than one host

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Can live outside host due to encystations that
allows formation of cyst enclosed in protective
capsule and when protozoan is in host, excystation
allows reproduction
After reproduction in host, protozoa uses hosts cells
for reproduction and feeding leading to cell
damage and death
Reproduction rate for protozoa is rapid but much
less than that of bacteria
Diseases include malaria, giardia, trichomoniasis,
toxoplasmosis
iii. MULTICELLULAR ORGANISMS
- Two types associated with infectious diseases: fungi and
helminthes
- Many fungi are unicellular but can exist as multicellular
a. Fungi
Include unicellular yeasts and molds (can be unicellular but
mostly multicellular)
Reside in soil
Inhaled as spores
Exception is candida, yeast that exists as a
component of the normal body flora (when
immune response is compromised, candida can
reproduce to become a pathological infection)
Can exist in feces such a fungus found in bird and bat
feces which can cause histoplasmosis
Can be mutagenic
Aflatoxin is produced by some fungi and can
contaminate grains and nuts stored under less
than optimal conditions
Associated with liver cancer
b. Helminthes
Species of worms
Only rudimentary digestive and locomotor system (difficulty
surviving if not for highly adaptive parasitic nature can
live whole life in host and take advantage of its digestion)
Have developed reproduction system (transmitted as eggs
or larvae)
Two types:
Roundworms (trichinella which cause trichinellosis
or enterobius which causes pinworm)
Flatworms (tapeworms)
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