Disease-Epidemiology.pdf

Epidemiology
Epi = upon
Demos = people
Logos = science
"the study of what is (only) upon the people".
According to WHO (2017) „…. epidemiology is the study of the distribution and
determinants of health-related states or events (including disease), and the
application of this study to the control of diseases and other health problems.….‟

Epidemiology: Terminology
is the invasion of an organism's body tissues by disease-causing agents,
their multiplication, and the reaction of host tissues to these organisms and
the toxins they produce.
Infection
transmissible disease or communicable disease, is illness resulting from an
infection.
Infectious disease
A disease is a particular abnormal condition, a disorder of a
structure or function, that affects part or all of an organism.
Disease
is the rapid spread of infectious disease to a large number of people in a
given population within a short period of time, usually two weeks or less.
Epidemic
study of the interaction of diseases in a population
Syndemic
Inherited, acquired, or induced resistance to infection by a specific pathogen
Immunity

Epidemiology: Application/Scope
To determine, describe, and report on the natural course of disease, disability, injury, and death
To aid in the planning and development of health services and programs
To provide administrative and planning data
To study the cause (or etiology) of disease(s), or conditions, disorders, disabilities, etc.
To determine the primary agent responsible or ascertain causative factors
To determine the characteristics of the agent or causative factors
To determine the mode of transmission
To determine contributing factors
To identify and determine geographic patterns

Epidemiology: Types / Methods
• examining the distribution of disease in a
population, and observing the basic features of
its distribution
Descriptive epidemiology
• investigating a hypothesis about the cause of
disease by studying how exposures relate to
disease
Analytic epidemiology
Descriptive epidemiology is antecedent to analytical epidemiology.
• know where to look
• know what to control for
• develop viable hypotheses
Analytical epidemiology
studies require
information to ...

Epidemiology: Descriptive Epidemiology
Three essentials characteristics of disease that we look for in descriptive epidemiology are ….
Person
Place
Time
5W
What = health
issue of
concern
Who = person
Where = place
When = time
Why/how =
causes, risk
factors, modes
of transmission

• age, gender, ethnic group
• genetic predisposition
• concurrent disease
• diet, physical activity, smoking
• risk taking behavior
• SES, education, occupation
Person
• presence of agents or vectors
• climate
• geology
• population density
• economic development
• nutritional practices
• medical practices
Geographic Place

• Calendar time
• Time since an event
• Physiologic cycles
• Age (time since birth)
• Seasonality
• Temporal trends
Time

Three essentials characteristics of disease that we look for in analytic epidemiology are ….
Key feature of analytic epidemiology = Comparison group
• Demographic factor such as age, race, or sex;
• Constitutional factor such as blood group or immune
status;
• Behavior or act such as smoking or having eaten salsa;
or
• Circumstance such as living near a toxic waste site.
Characteristic
Identifying factors associated with disease help health officials appropriately target public health
prevention and control activities
It also guides additional research into the causes of disease.

Epidemiology: Epidemiological Studies
• investigator determines through a controlled process the
exposure
• tracks the individuals or communities over time to detect
the effects of the exposure.
• For example, in a clinical trial of a new vaccine
• Follow-up
Experimental
• Observes the exposure and disease status of each
study participant.
• Examples: Cholera in a place observational studies
Observational

Epidemiology: Epidemiological Studies
• Study on each participant is exposed or not
• Tracks the participants to see if they develop the disease of
interest
Cohort studies
• A group of people with disease
• Comparison group to compare previous exposures between
the two groups
Case-control studies
• A sample of persons from a population
• Tends to assess the presence (prevalence) of the health
outcome
• For example, in a cross-sectional study of diabetes
Cross-sectional studies
Types of Observational Studies

Epidemiology : The Epidemiologic Triangle
The epidemiologic triangle is made up of three
parts: agent, host and environment (there are
three factors to study the causes of a disease, in
analytic epidemiology:
• Agent
• Host
• Environment
Epidemiologic
Triangle
is a model or tool for studying health problems
i.e., how a disease spreads and how to combat it.
Time

• that causes the disease (the “what” of
the Triangle)
• The agent is the microorganism that
actually causes the disease in question.
• An agent could be some form of
bacteria, virus, fungus, or Protozoa
(parasite).
Agent
• are single-celled organisms.
• Capable to reproduce
themselves, by themselves.
• They are larger than viruses
(but still much too small to be
seen with the naked eye).
• They are filled with fluid and
may have threadlike structures
to move themselves, like a tail.
Bacteria
• A virus may have a spiny
outside layer, called the
envelope.
• Viruses have a core of
genetic material, but no way
to reproduce it on their own.
• Viruses infect cells and take
over their reproductive
machinery to reproduce.
Virus
• 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.
Fungi
• Protozoa are very small
• Most live in water
• They are parasites, which
means they live off other
organisms, in some cases
humans.
• Malaria is a parasitic
protozoan, as is Giardia.
Protozoa
• Types of Agents
• Biological (micro-organisms)
• Physical (temperature, radiation, trauma, others)
• Chemical (acids, alkalis, poisons, tobacco, others)
• Environmental (nutrients in diet, allergens, others)
• Psychological experiences, stress, crime

• organism harboring the disease (the “who” of the Triangle)
• The agent infects the host, which is the organism that carries the
disease.
• A host doesn‟t necessarily get sick; hosts can act as carriers for an
agent without displaying any outward symptoms of the disease.
• Hosts get sick or carry an agent because some part of their
physiology is hospitable or attractive to the agent.
• The “host” heading also includes symptoms of the disease.
• Different people may have different reactions to the same agent.
• For example, adults infected with the virus varicella (chickenpox)
are more likely than children to develop serious complications.
Host

• those external factors that cause or allow disease transmission
(the “where” of the Triangle)
• The environment includes any factors that affect the spread of
the disease but are not directly a part of the agent or the host.
• Some diseases live best in dirty water. Others survive in human
blood.
• For example, the temperature in a given location might affect an
agent‟s ability to thrive, as might the quality of drinking water or
the accessibility of adequate medical facilities.
• Still others, like E. coli, thrive in warm temperatures but are
killed by high heat. Other environment factors include the
season of the year
Environment

• In the center of the Triangle is time.
• Most infectious diseases have an incubation period—the time
between when the host is infected and when disease symptoms
occur.
• Time may describe the duration of the illness or the amount of
time a person can be sick before death or recovery occurs.
• Time also describes the period from an infection to the threshold
of an epidemic for a population.
Time
Epidemics occur when… host, agent and environmental factors are not in balance.
• due to new agent
• due to change in existing
• agent (infectivity, pathogenicity, virulence)
• Due to change in number of susceptible in the population
• Due to environmental changes that affect transmission of the agent of growth of the agent

Epidemiology : The Epidemiologic Triangle Example-1
HIV
AGENT HOST ENVIRONMENT
• HIV is a viral infection that
targets a person‟s immune
system, making it more
vulnerable to other forms of
infection.
• Because the virus targets the
immune system itself, the body
cannot effectively fight HIV on
its own.
• HIV is communicated through
direct contact with an infected
person‟s bodily fluids, and it
primarily spreads through sexual
contact or shared needles.
• HIV was originally carried by
chimpanzees and that humans
who hunted these chimpanzees
for meat became infected with a
mutated form of the virus upon
contact with the chimpanzees‟
blood.
• HIV can be transmitted when a
bodily fluid such as blood comes
into contact with a mucous
membrane or damaged tissue
(such as an open wound or the
mucous membranes found inside
the mouth).
• There are a number of socio-
economic factors that can impact
the spread of HIV within a
community.
• Communities with higher
concentrations of sexually
transmitted diseases (STD) and
lower incidences of reporting due
to social pressure or otherwise
allow HIV to flourish.
• Poverty limits access to care and
treatment, and discrimination can
discourage individuals from
being tested or seeking care.

Epidemiology : The Epidemiologic Triangle Example-2
Smoking Related Disease
AGENT HOST ENVIRONMENT
• Unlike the agent in many
epidemiologic triangles,
cigarettes and other forms of
tobacco are not microbes, and
smoking is not contagious in
the usual sense.
• It is not transmitted from
person to person,
though second-hand smoke is
dangerous.
• However, a carcinogen in the
smoke of a cigarette is an
agent.
• People become a potential host
for smoking-related diseases
when they smoke cigarettes or
when they inhale second-hand
smoke.
• A host can then suffer a variety
of ailments, from lung cancer to
heart disease to diabetes.
• Not all people who smoke
suffer the same effects at the
same rates; genetics play a part
here, as do environmental
factors.
• The reasons people smoke are almost
entirely social.
• Pressure from peers or friends, a need to
fit in, or susceptibility to the marketing
efforts of tobacco companies are all
factors that can lead to smoking and,
consequently, smoking-related diseases.
• In addition, environmental factors such as
the frequency at which people smoke and
the length of time they remain a smoker
can affect their chances of contracting a
smoking-related disease.
• Because cigarettes and other tobacco
products are highly addictive, individuals
are likely to continue to smoke once they
start.

Epidemiology: Disease
The study of disease is called pathology which includes the study of cause.
Disease is associated with specific symptoms and signs.
It may be caused by external factors such as pathogens, or it may be caused by internal
dysfunctions.
Does not allow the body to function normally.
Can affect individual organs or an entire body system.

Epidemiology: Types of Disease
• Acute diseases are those conditions in which the peak severity of
symptoms occurs within three months (usually sooner), and recovery in
those who survive is usually complete
• Example: Asthma attack; Broken bone; Bronchitis; Burn; Common
cold; Flu; Heart attack; Pneumonia; Respiratory infection; Strep throat
Acute Diseases
• Chronic diseases or conditions are those in which symptoms continue
longer than three months and in some cases for the remainder of the
person‟s life. Recovery is slow and sometimes incomplete.
• Examples: Alzheimer‟s disease; Arthritis; Chronic obstructive
pulmonary disease (COPD); Depression; Diabetes; Heart disease; High
blood pressure; High cholesterol; Obesity; Osteoporosis; Stroke
Chronic Diseases
Based on symptom /effect appearance

Based on symptom /effect appearance
Acute Chronic
Definition An acute disease is a disease
with a rapid onset and/or a short
course.
A chronic condition is a human health condition or disease
that is persistent or otherwise long-lasting in its effects.
Appearance of
symptoms
Sudden Usually gradual
Duration Short; a few days to a week or
two.
Extended period of time; usually six weeks or more, often
months or years.
Nature of Pain Starts suddenly as a reaction to
an injury or something else.
Develops gradually out of habitual diet, posture or other
condition. Continues beyond expected period of recovery.
Examples Breaking a bone, burn, strep
throat, flu, asthma
attack, heartburn.
Osteoporosis, asthma, frequent migraines, consistent back
pain, heart disease, kidney disease.

Based on Transmission
1.
Communicable
(Infectious)
Diseases
Diseases for which biological agents or their products are the cause and
which are transmissible from one individual to another
The disease process begins when the causative agent is able to lodge and
grow or reproduce within the body
The process of lodgment and growth of a microorganism or virus in the
host is termed infection
Examples: Carbapenem-Resistant Enterobacteriaceae (CRE); Ebola;
Enterovirus D68; Flu; Hantavirus; Hepatitis B; HIV/AIDS; Measles;
Methicillin-resistant S. Aureus (MRSA); Pertussis; Rabies; Sexually
Transmitted Disease; Shigellosis; Tuberculosis; West Nile Virus; Zika.

Based on Transmission
2. Non-
communicable
(Noninfectious)
Diseases/Illnesses
Those diseases or illnesses that cannot be transmitted from an infected
person to a susceptible, healthy one
Several, or even many, factors may contribute to the development of a
given non-communicable health condition
The contributing factors may be genetic, environmental, or behavioral
in nature
Examples: Cancer; Diabetes; Hypertension; Osteoporosis; Alzheimer‟s;
Heart Disease; Fibromyalgia

Acute Diseases
Communicable Common cold, pneumonia, mumps, measles, pertussis, typhoid
fever, flu
Non-communicable Appendicitis, Poisoning, Trauma (e.g., due to automobile
accidence, fires, etc.)
Chronic Diseases
Communicable Lyme disease, Tuberculosis, AIDS, Syphilis, rheumatic fever
following Streptococcal infections, Herpes
Non-communicable Diabetes, coronary heart disease, osteoarthritis, cirrhosis of the
liver due to alcoholism, hypertension

Epidemiology: Path/Way/Course Infectious of Disease
• Invasion of Host
Exposure
• period of time between exposure and onset of
symptoms -- e.g., interval between HIV infection
and development of AIDS can be as long as 10-15
years
Incubation
• Disease runs course -- treatment, recovery/death
(most people don‟t die from infectious diseases)
Host reaction
Some Terminologies

• Varies by disease
• Salmonella -- 12-72 hours after infection; symptoms
usually resolve in 5-7 days, unless infected person is
in a very weakened health status
• Measles (rubeola) -- approx. 10-12 days (prodomal -
- i.e., interval between the earliest symptoms and the
appearance of the rash or fever -- rash onset, on
average, 14 days
• HIV -- 6 weeks upward to months; interval between
HIV infection and development of AIDS can be as
long as 10-15 years
• 2-6 weeks after infection in many, but not all,
diseases, most people develop antibodies against
reinfection
INCUBATION PERIOD
Epidemiology: Path/Way/Course Infectious of Disease

Infection is the invasion and growth of bacteria, viruses, or parasites that are not
typically existing within the body.
For an infection to develop, each link of the chain must be connected.
Breaking any link of the chain can stop the transmission of infection!
The spread of infection follows a
strict chain consisting of six
elements / stages.
Epidemiology: Chain/Path/Way/Course Infectious of Disease

Stage 1: INFECTIOUS DISEASE
• Any microorganism that can cause a disease such as a bacterium,
virus, parasite, or fungus. Reasons that the organism will cause an
infection are virulence (ability to multiply and grow), invasiveness
(ability to enter tissue), and pathogenicity (ability to cause
disease).
Stage 2: RESERVOIR
• The place where the microorganism resides, thrives, and
reproduces, i.e., food, water, toilet seat, elevator buttons, human
feces, respiratory secretions.

Stage 3: PORTAL OF EXIT
• The place where the organism leaves the reservoir, such as the
respiratory tract (nose, mouth), intestinal tract (rectum), urinary
tract, or blood and other body fluids.
Stage 4 : MODE OF TRANSMISSION
• The means by which an organism transfers from one carrier to
another by either direct transmission (direct contact between
infectious host and susceptible host) or indirect transmission
(which involves an intermediate carrier like an environmental
surface or piece of medical equipment).

Stage 5: PORTAL OF ENTRY
• The opening where an infectious disease enters the host‟s body such
as mucus membranes, open wounds, or tubes inserted in body
cavities like urinary catheters or feeding tubes.
Stage 6: SUSCEPTIBLE HOST
• The person who is at risk for developing an infection
from the disease. Several factors make a person more
susceptible to disease including age (young people and
elderly people generally are more at risk), underlying
chronic diseases such as diabetes or asthma, conditions
that weaken the immune system like HIV, certain types of
medications, invasive devices like feeding tubes, and
malnutrition.

• Immediate transfer of the disease agent by direct contact
between the infected and the susceptible individuals
• Occurs through such acts as touching, biting, kissing,
sexual intercourse, or by direct projection (droplet
spread) by coughing or sneezing within a distance of one
meter
• Examples of diseases for which transmission is usually
direct are AIDS, syphilis, gonorrhea, and the common
cold
Direct Transmission
Epidemiology: Mode of Communicable Disease Transmission
• May be one of three types:
• air-borne
• vehicle-borne
• vector-borne
Indirect
Transmission

Indirect Transmission
• transmission of microbial aerosols to a suitable port of
entry, usually the respiratory tract
• Microbial aerosols are suspensions of dust or droplet
nuclei made up wholly or in part by microorganisms --
may be suspended and infective for long periods of
time
• Examples of air-borne diseases include tuberculosis,
influenza, histoplasmosis, and legionellosis
Air-borne
transmission
• contaminated materials or objects (fomites) serve as vehicles, nonliving
objects by which communicable agents are transferred to a susceptible host
• The agent may or may not have multiplied or developed on the vehicle
• Examples of vehicles include toys, handkerchiefs, soiled clothes, bedding,
food service utensils, and surgical instruments
• Also considered vehicles are water, milk, food (e.g., common vehicles), or
biological products such as blood, serum, plasma, organs and tissues
• Almost any disease can be transmitted by vehicles, including those for which
the primary mode of transmission is direct, such as dysentery and hepatitis
Vehicle-borne
transmission

• disease transfer by a living organism, such as a mosquito, fly, or tick
• Transmission may be mechanical, via the contaminated mouth parts
or feet of the vector, or biological, involving multiplication or
developmental changes of the agent in the vector before
transmission occurs
• In mechanical transmission, multiplication and development of the
disease do not usually occur -- e.g., organisms that cause dysentery,
polio, cholera, and typhoid fever have been isolated from such
insects as cockroaches and house flies and could presumably be
deposited on food prepared for human consumption
Vector-borne
transmission

Mosquitoes
• are extremely important vectors of human diseases -- e.g., they transmit the viruses that cause yellow
fever and dengué fever as well as 200 other viruses -- they also transmit malaria, which infects 100
million people in the world each year (most in tropical areas), killing at least 1 million of them each year
Ticks
• are another important biological vector, transmitting Rocky Mountain spotted fever, relapsing fever, and
Lyme disease
In biological transmission
• multiplication and/or developmental changes of the disease agent occur in the vector before
transmission occurs
• Biological transmission is much more important than mechanical transmission in terms of its
impact on public/community health
• Examples of biological vectors include mosquitoes, fleas, ticks, lice, flies and other insects

Outcomes of Infection
• (i) infection without illness;
• (ii) infection with illness; (with or
without long-term sequelae) and
• (iii) infection, illness and then death
Microbes differ in their
ability to produce the
different outcomes of
infection:

Outcomes
• all cause / cause-specific
Mortality
• Disease-specific indicators / General indicators: clinic
use, hospitalization, medication use
Morbidity
• General / Disease-specific
Quality of life
Costs

The Iceberg Concept: As Applied to Virus Infections

Epidemiology: Iceberg of Disease
1 Diseased, diagnosed & controlled
2 Diagnosed, uncontrolled
3 Undiagnosed or wrongly
diagnosed disease
4 Risk factors for disease
5 Free of risk factors
Diagnosed
disease
Undiagnosed or
wrongly diagnosed disease
The concept of the "iceberg phenomenon of disease "gives an idea
of the progress of a disease from its sub-clinical stages to overt or
apparent disease state.

Epidemiology: Iceberg of Disease
The submerged portion of the iceberg represents the hidden mass of the disease (e.g., subclinical cases,
carriers, undiagnosed cases).
The floating tip represents what the physician sees in his practice/chamber/hospital etc. and are
recognized as cases (diseased persons).
The vast submerged portion of the iceberg represents the hidden mass of the disease that is unrecognized/
latent/ inapparant / pre-symptomatic/ undiagnosed cases and carriers in the community.
The water line represents the demarcation between clinical and subclinical or undiagnosed patients.
In some cases (for example, hypertension, diabetes, anemia, malnutrition and mental illness) the
unknown morbidity (represented by submerged portion of iceberg) far exceeds the known morbidity.
Thus detection and control undiagnosed reservoir of disease is a challenge to modern technique.

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