Download Lecture 17

PART 3: Microorganisms, Humans,
and Disease
•  Epidemiology and transmission of
disease
Epidemiology
•  Epidemiology is the study of disease in
populations, and includes the transmission,
incidence, frequency, and distribution of disease. •  The Centers for Disease Control in Atlanta is the
main source of information in the United States:
their publication “Morbidity and Mortality
Weekly” tells us how many people are sick and
how many have died from various diseases (light
reading with coffee in the morning).
•  They are also responsible for tracking and
identifying emerging diseases, and report their
findings to the World Health Organization.
Disease occurrence
•  Incidence of a disease refers to the
percentage of the population that gets the
disease during a given time period;
•  Prevalence refers to the percentage of a
population which has the disease during a
given time period.
Virulence of a pathogen
•  Virulence refers to the DEGREE of pathogenicity
for a given pathogen.
•  Virulence can be measured as the LD50 (lethal
dose for 50% of inoculated hosts) or the ID50
(infectious dose for 50% of inoculated hosts)
•  The ID50 can range from literally 1 cell to
1 billion cells!
– (some pathogens are more able to cause
disease, that is they are more virulent, than
others)
More terms
•  A disease is said to be communicable if it
can be transmitted from one host to another.
–  A contagious disease is very easily
communicable. Contrast AIDS w/ the flu.
•  A disease is said to be non-communicable
if it cannot be transmitted from one host to
another, usually because it is acquired from
the environment (e.g. botulism)
Frequency of occurrence
•  Sporadic - occurs only occasionally in a
population (e.g. typhoid fever)
•  Endemic - constantly present in a population
(e.g. the common cold)
•  Epidemic - acquired by many people in a given
area over a short time (often the flu); AIDS?
•  Pandemic - epidemic worldwide - AIDS?
The Spread of Infection
•  A reservoir is a continual source of infective
pathogens. Reservoirs may be –  Human - people infected with the pathogen;
–  Animal - zoonoses are diseases that can be transmitted
from animals to humans (rabies); also, vectors like
mosquitoes are considered animal reservoirs;
–  Non-living - soil or water; the pathogen can exist
stably outside a host (B. anthracis because it forms
endospores; Vibrio cholerae [causes cholera] often
persists in water contaminated from feces)
Disease Transmission (careful, parts of this are a little tricky!)
•  Contact transmission:
–  Direct contact: transmission of a pathogen by physical contact
between living reservoir and host; no intermediate object is involved.
Ex. Touching (the common cold), kissing (infectious
mononucleosis), sexual intercourse (HIV)
–  Indirect contact: transmission of a pathogen from a living reservoir
to a host via an intermediate object, called a FOMITE. Example:
HIV transmission via sharing hypodermic needles.
–  Droplet transmission: transmission of a pathogen from a living
reservoir to a host via droplets of mucus which travel less than 1
meter via sneezing, coughing, etc. (the flu) Disease Transmission •  Vehicle transmission: transmission
of a pathogen by
physical contact between NON-LIVING reservoir
and host.
•  Non-living reservoirs are often food, water, or air.
–  Foodborne transmission - foods that are incompletely
cooked or unsanitary (Salmonella poisoning, trichinosis)
–  Waterborne transmission - water is often contaminated by
poorly treated sewage (cholera)
–  Airborne transmission: transmission is considered airborne
if the pathogen can travel more than 1 meter from the host
(tuberculosis)
PLEASE: Recognize the
difference between
vehicle transmission and
indirect contact!
Arthropod vectors: Biological
transmission
•  A special form of direct contact in which an
arthropod harbors a pathogen and transmits
it from host to host. We discussed malaria
as an example, also Lyme disease.
Etiology - the cause of disease
•  Koch's postulates: Robert Koch was the first
microbiologist to come up with a set of rules to
determine which microorganism caused which
disease.
•  Koch showed that Bacillus anthracis causes
anthrax and that Mycobacterium tuberculosis
causes tuberculosis, among others.
•  What did he say?
Koch's Postulates
•  The same pathogen must be present in every case
of the disease;
•  The pathogen must be isolated from diseased host
in pure culture;
•  The cultured pathogen must be capable of causing
the disease in healthy lab animals;
•  The pathogen must be re-isolated from the
inoculated lab animal.
Potential Exceptions to Koch's
Postulates
•  Viruses and certain bacteria cannot be cultured
normally;
•  Some diseases (pneumonia) may be caused by a variety
of different pathogens;
•  Some pathogens may be able to cause several different
diseases (S. pyogenes);
•  Certain pathogens cause diseases in humans only, so
would not infect lab animals (no, you can't experiment
with humans yet!). HIV is an example.