2. Terms
• Disease – a pathological condition of body parts
or tissues characterized by an identifiable group
of signs and symptoms.
• Infectious disease – disease caused by an
infectious agent such as a bacterium, virus,
protozoan, or fungus that can be passed on to
others.
• Infection – occurs when an infectious agent or
pathogenic organism enters the body and begins
to reproduce; may or may not lead to disease.
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3. • Pathology: scientific study of disease
• Pathogen – an infectious agent that causes
disease.
• Host – an organism infected by another organism.
• Virulence – the relative ability of an agent to
cause rapid and severe disease in a host
• Etiology: causative agent of a disease
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4. Concept of Disease
• Webster Dictionary “A condition in which body
function is impaired, departure from a state of
health, an alteration of the human body
interrupting the performance of the vital functions
• Oxford English Dictionary – the condition of
body or some part of organ of body in which its
functions are disrupted or deranged (unbalanced).
• Ecologically – ‘a maladjustment of human
organism to the environment’
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5. Concept of causation
• Germ theory
• Epidemiological Triad
• Multi-factorial causation
• Web of causation
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6. Germ theory
• the theory stated that all infectious diseases are
caused by microorganisms
•
• These small organisms, too small to see
without magnification, invade humans,
animals, and other living hosts.
• Their growth and reproduction within their
hosts can cause a disease.
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7. Epidemiological triad
• The Epidemiologic Triangle is a model that
scientists have developed for studying health
problems.
• The Triangle has three corners; agent, host
and environment
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9. 1-Agent, or microbe that causes the disease
• the agent is a microbe—an organism too small to
be seen with the naked eye that cause the disease.
•
• Disease-causing microbes are;
• bacteria,
• virus,
• fungi,
• protozoa
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10. Bacteria
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Bacteria are single-celled
organisms. Bacteria have the
tools to reproduce themselves,
by themselves. They are larger
than viruses
They are filled with fluid and
may have threadlike structures
to move themselves, like a
tail.
11. Virus
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Viruses have no nucleus,
cytoplasm, organelles, or
cell membrane, so can not
carry out cellular functions.
Only able to replicate by
infecting cells and using the
organelles and enzymes
very small, size
Consists of two parts: a nucleic
acid and a protein coat called a
capsid
Nucleic acid may be DNA or
RNA but not both.
12. HIV virus
(human immunodeficiency virus).
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AIDS (Acquired Immune Deficiency
Syndrome) is the disease caused by the
virus called HIV (human
immunodeficiency virus).
HIV attacks cells of the immune system
and destroys their ability to fight infection
by other agents.
HIV is spread through the direct
exchange of body fluids.
There is a long period of time from HIV
infection to the onset of AIDS.
Anti-HIV drugs prolong the length and
quality of life, but there is no vaccine or
cure for AIDS.
14. Fungi
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Fungi are like plants
made up of many cells.
They are not called
plants because they
cannot produce their
own food from soil and
water. Instead, they live
off animals, including
people, and plants.
Mushrooms and yeast
are fungi.
15. Protozoa
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Protozoa are very
small. Most live in
water. They are
parasites, which means
they live off other
organisms. Malaria and
Giardia are parasitic
protozoa.
16. 2- Host
• Hosts are organisms, usually humans or
animals, which are exposed to disease.
• The host can be the organism that gets sick, as
well as any animal carrier (including insects
and worms) that may or may not get sick.
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17. Types of Host Factors
• Physiological
• Anatomical
• Genetic
• Behavioral
• Occupational
• Constitutional
• Cultural
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18. Routes that microbes use to exit host
• Respiratory tract: via cough/sneeze
• Gastrointestinal tract GI tract: via feces
• Urogenital: via vaginal/penile secretions
• Skin: via open wounds
• Blood: open wounds, surgery, syringes
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19. 3-Environment, or those external factors that cause
or allow disease transmission.
• Favorable surroundings and conditions
external to the host that cause or allow the
disease to be transmitted.
• Some diseases live best in dirty water. Others
survive in human blood. Still others, like E.
coli, thrive in warm temperatures but are
killed by high heat.
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20. Types of Environmental Factors
• Physical, chemical, biological
• Social, political, economic
• Population density
• Cultural
• factors that affect presence and levels of
agents
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22. 3-Multi-factorial causation
• Is known as modern disease of civilization such
as; lung cancer, coronary heart disease, mental
illness etc
• these disease is not explained on the basis of the
germ theory and could not be prevented by;
• isolation,
• immunization
• improvement in sanitation
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24. 4-Web of causation
• The web of causation is primarily a medical
term used to represent the complex group of
subjects and relationships that can contribute
to the occurrence and spread of a disease.
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web of causation, an interrelationship of multiple
factors that contribute to the occurrence of a disease
26. Natural history of diseases
• Natural history of disease refers to the progress
of a disease process in an individual over time,
in the absence of intervention.
• The process begins with exposure to or
accumulation of factors capable of causing
disease.
• Without medical intervention, the process ends
with recovery ,disability, or death
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28. • Pre pathogenicity
• Pre-pathogenesis is before the beginning or
onset of a disease
• pathogenicity
• the origin and development of the disease, and
whether it is acute, chronic, or recurrent.
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29. • acute disease; a disease characterized by a
relatively sudden onset of symptoms that are
usually severe, Examples are pneumonia and
appendicitis
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30. • Chronic disease: A disease that persists for a long
time. A chronic disease is one lasting 3 months or
more, by the definition of the U.S. National
Center for Health Statistics. Chronic diseases
generally cannot be prevented by vaccines or
cured by medication, nor do they just disappear.
Example such as cardiovascular disease such as
heart attacks and stroke, cancer such as breast and
colon cancer, diabetes
• Recurrent disease; Occurring or appearing again
or repeatedly.
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31. • Types of pathogenesis include;
• microbial infection,
• inflammation,
• malignancy and tissue breakdown.
• For example bacterial pathogenesis is the
mechanism by which bacteria cause infectious
illness
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32. Predisposing Factors of Disease
• Makes the body more susceptible to disease or
alters the course of the disease
• gender
• age
• fatigue
• climate
• poor nutrition
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33. Progression of disease
• An abnormal condition of a part, organ, or
system of an organism resulting from various
causes, such as infection, inflammation,
environmental factors, or genetic defect,
• and characterized by an identifiable group of
signs, symptoms, or both.
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34. Progression of disease in an individual over
time
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Four common stages that most disease manifests
35. Four common stages that most disease
manifests
• Stage of susceptibility ;
• susceptible due to the presence of risk factors.
Individuals with high serum cholesterol,
• hypertension, a sedentary lifestyle, and
diabetes, for example, have an increased risk
of developing coronary heart disease.
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36. • Stage of pre-symptomatic disease; or stage
of subclinical disease
• the disease process has begun, but no overt
signs or symptoms are evident to the host.
• For non-communicable diseases, this stage
includes the incubation period, (the time
between the invasion of an infectious agent
and the development of the first signs or
symptoms of the disease)
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37. • Stage of clinical disease
• a stage in the history of a pathologic condition
that begins with anatomic or physiologic
changes that are sufficient to produce
recognizable signs and symptoms of a disease.
•
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38. • Stage of diminished capacity or Stage of
recovery, disability or death
• A reduced capacity, caused by mental illness,
intoxication, or some other cause of diseases,
that prevented a person from normal activities
• Or recovery from disease
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39. infectious diseases
• Infectious diseases are disorders caused by
organisms — such as bacteria, viruses, fungi
or parasites.
• infectious diseases result from the interaction
of agent, host, and environment
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40. infectious diseases
• Some infectious diseases can be passed from
person to person.
• Some are transmitted by bites from insects or
animals.
• And others are acquired by ingesting
contaminated food or water or being exposed
to organisms in the environment.
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41. infectious diseases
• Signs and symptoms vary depending on the
organism causing the infection, but often
include fever and fatigue.
• Mild complaints may respond to rest and
home remedies, while some life-threatening
infections may require hospitalization.
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42. infectious diseases
• Many infectious diseases, such as measles and
chickenpox, can be prevented by vaccines.
• Frequent and thorough hand-washing also
helps protect you from infectious diseases.
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43. Severity of Infectious Disease
• Infectivity refers to the proportion of exposed
persons who become infected.
• Measures of Infectivity,
• Infectivity (ability to infect)
• (number infected / number susceptible) x
100
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44. • Pathogenicity refers to the proportion of
infected persons who develop clinical disease.
• Pathogenicity (ability to cause disease)
• (number with clinical disease / number
infected) x 100
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45. • Virulence refers to the proportion of persons
with clinical disease who become severely ill
or die
• Virulence (ability to cause death)
• (number of deaths / number with disease) x
100
• All are dependent on host factors
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46. Classifying Infectious Diseases
• Communicable Diseases: transmitted directly
or indirectly from one host to another
• Contagious Diseases: easily spread from one
person to another
• Non-communicable Diseases: not spread
from host to another
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47. Stages of infectious diseases
stages of infectious
diseases
Definition
incubation time between infection, signs and
symptoms
prodromal mild symptoms
invasive normal signs and symptoms
Acme signs and symptoms at their
strongest
decline death and defeat
convalescence healing
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48. Classification of Disease
Based on frequency of occurrence:
• Sporadic; appearing or happening at irregular
intervals in time; occasional:
• Endemic; Prevalent in or limited to a
particular locality, region, or people
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49. Classification of Disease
Based on frequency of occurrence:
• Epidemic; a widespread occurrence of an
infectious disease in a community at a
particular time.
• Pandemic; prevalent over a whole country
or the world.
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51. chain of infection
• infectious diseases result from the interaction
of agent, host, and environment.
• transmission occurs when the agent leaves its
reservoir or host through a portal of exit, and
is carried by some mode of transmission, and
enters through an appropriate entry to infect
a susceptible host.
• This is sometimes called the chain of infection
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52. 6 links in the chain of infection
• The Infectious Agent; -any disease-causing
microorganism (pathogen)
• The Reservoir Host; -the organism in which
the infectious microbes reside
• The Portal of Exit; route of escape of the
pathogen from the reservoir
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53. • The Route of Transmission; method by which
the pathogen gets from the reservoir to the
new host
• The Portal of Entry; -route through which the
pathogen enters its new host
• The Susceptible Host; -the organism that
accepts the pathogen
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54. references
• Park's Textbook of Preventive and Social
Medicine 21 Edition, by K. PARK
• World Health Organization reports
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