Lec 5 introduction to demography

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WEEK 5: INTRODUCTION TO THE PRINCIPLES OF EPIDEMIOLOGY
– Defining Epidemiology
– Essential Components
– Objectives of Epidemiology
– Concepts of Disease Occurrence
– Core Epidemiological Functions

LECTURE

DEFINING EPIDEMIOLOGY
•

Epidemiology

ESSENTIAL COMPONENTS OF EPIDEMIOLOGY
Disease
Distribution

how cases of condition of interest spread/distributed
EX: gender, age, geographic location, socioeconomic status

Disease
Determinants

What risk factors or antecedent events are
associated with the appearance of a disease or
condition

Disease
Frequency

Number of cases of the condition occurring over a
given time period

– Epi: on or upon
– Demos: people
– Logos: study of
– study of the distribution and determinants of health-related states

❖

or events in specified populations, and the application of this study
to the control of health problems (A Dictionary of Epidemiology;
Last, JM et al, 1988)

– systematically gathering of info about health events
Who is sick?
What are their symptoms?
When did they get sick?
Where could they have been exposed to the illness?
✓ Using statistical analysis, investigators study the answers to these
questions to find out how a particular health problem was introduced into
a community
✓ Epidemiology is concerned with the overall health of the people and how
to mitigate various diseases

Study

*Prognosis – effectiveness of treatment and levels of prevention done to
mitigate a disease

Epidemiology applies the basic science of public health
by applying highly quantitative discipline based on
principles of statistics and research methodologies

Distribution

APPLIED IN: Descriptive Epidemiology
Distribution of frequencies and patterns of health
events (person, place, time) within groups in a
population where the health events took place/ will
happen

Determinants

– search for causes or factors that are associated with
increased risk or probability of disease
– uncovered and revealed thru Analytical Epidemiology
– seek answers to who, what, where, when, how and
why health events happened

CONCEPTS OF DISEASE OCCURRENCE
✓
✓

✓

Infectious diseases, chronic disease, environmental
Health-Related
problems, behavioral problems, and injuries /// issues
States
found where actions and mitigations are needed
Populations

Groups of people rather than with individual patients

Control

APPLIED IN: Applied or Field Epidemiology
– based on the epidemiologic data results to public
health decision making and aids in developing and
evaluating interventions to control and prevent
health problems

Objectives of Epidemiology
✓ Identify the etiology or the cause of a disease and the risk
factors
✓ To determine the extent of disease found in the community
and set up priorities for interventions
✓ To study the natural history and prognosis of disease
✓ To evaluate public health intervention, policies, and modes of
health care delivery
✓ To provide the foundation for developing public policy and
regulatory decisions relating to environmental problem
✓ To communicate the findings to health professionals and
the public

Diseases are more likely to occur if infectious agents are present
Disease and other health events do not occur randomly in a
population, but are more likely to occur in some members of the
population than others because of risk factors that may not be
distributed randomly in the population
DISEASE CAUSATION: Epidemiologic Triad/Triangle

EPIDEMIOLOGIC TRIANGLE

– model of disease causation

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▪ Three Characteristics that are Examined to Study the Cause/S For
Disease in Analytic Epidemiology

❖ Response to Exposure
No effect
Good immune system
Accumulation of symptoms but you
Subclinical Disease
are not yet showing any of those
Your symptoms are unusual/ not
Atypical Symptoms
fitting a single diagnostic
Symptomatic Illness Common symptoms are very evident
Severe Illness
May lead to death/ worse diseases

Disease is the result of forces within a dynamic
system consisting of: Agent, Host, Environment

INFECTpIOUS AGENT
•

Agent

FAVORABLE ENVIRONMENT

– entity necessary to cause disease in a susceptible host

•

❖ Examples
✓ Biological
✓ Physical
✓ Chemical
✓ Nutrient
✓ Physical environment
✓ Biological environment
✓ Social environment
✓ Mechanical
✓ Social

Environment

– introduces the infectious agent to host
– conditions or influences that are not art of either the agent or the
host, but that influence their interaction

❖ Factors
o Physical = geological location, climate
o Biologic = genetic makeup, brain chemistry, hormonal levels
o Socio-economic = overcrowding, sanitation, availability of
health services

COMMUNICABLE DISEASES: Pathogens
o Biological – bacterium, parasite, or virus
❖ Characteristics of Biological Agents
Infectivity

capacity to cause infection in a
susceptible host; chances/rate of how
much it is infectious

Pathogenicity

capacity to cause disease in a host

Virulence

severity of disease that the agent
causes in the host

CHAIN OF INFECTION
•

Chain of Infection

– transmission occurs when the agent leaves its reservoir or host

through a portal of exit, is conveyed by some mode of transmission,
and enters through an appropriate portal of entry to infect a
susceptible host; disease transmission of infectious disease

NON-COMMUNICABLE DISEASES: Risk Factors
o Physical force – motor vehicle crashes
o Chemical – environmental problem
o Nutritional imbalance – rickets
SUSCEPTIBLE HOST
•

Host

– person or the organism who will contract the disease
– susceptible to the effect of the agent
❖ Risk Factors that makes a host susceptible
o Demographic characteristics = age, sex
o Biological characteristics = genetic composition, nutritional
needs
o Socioeconomic characteristics = capability for health
services, hygiene
*Risk Factors influence the individuals’ exposure
(behaviors and habits), susceptibility (genes, nutritional
needs, immunologic status), and response to diseases
❖ Status of the Host
a) Susceptible = immunodeficient PX
b) Immune
c) Infected

*Chain of infection depends on the Epidemiologic Triad
o Natural History and Spectrum of Disease – refers to the
progression of a disease process in an individual over time, in the
absence of treatment; onset of disease until death w/o treatment;

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disease process = onset progression of disease until it becomes
stable
✓ Onset of disease begins when there is sufficient accumulation of risk
factors & exposure to agents
✓ After onset of disease, pathologic changes may occur (visible or not)
which will then proceed as a clinical disease, disability, or death
o Stages of Diseases
o Stage of Susceptibility = period of exposure to pathogens/
accumulation of risk factors enough to exhibit pathological
changes
o Stage of Subclinical Disease = incubation period for infectious
diseases; latency period for chronic diseases
(noncommunicable); ends until symptoms occur = PX still
asymptomatic; application of secondary level of prevention
o Stage of Clinical Disease = beginning of the symptoms; when
symptoms are evident, disease is already stabilized in the host,
affecting our immune system; where most disease are classified
into stages (Stage eme eme sa cancer); application of tertiary
level of prevention
o Stage of Recovery, Disability, or Death = last stage of disease
process

*Transition from subclinical to clinical may or may not show evident
symptoms of disease bc of the immune system of host
*Spectrum of Disease – entire range of disease if level of prevention is not
applied at all; categorization of diseases from severe, fatal, mild, or
moderate types; ; gaano kalala yung sakit; tertiary level of prevention is
applicable

– habitat in which the agent normally lives, grows, and
multiplies
– may or may not be the source from which an agent
is transferred to a host.
* disease transmission begins when pathologic agent
leaves reservoir thru portal of exit

Reservoir

CAN BE:
o Environment
– transmitted from plants, soil (Clostridium tetani =
tetanus), and water (Legionella pneumophila =
Legionnaire’s disease) in the environment
o Animals
– transmitted from animal to animal, with humans as
incidental hosts (dead end host = human can
harbour pathogen but incapable of transmitting
pathogen to another; sila na last)
examples:
✓ birds = psittacosis, Newcastle disease (by
para-myxo virus; infect wild and domestic birds
via aerosol transmission >> transmitted to
humans via contaminated food, water,
equipment), west nile virus
✓ bats = henda virus, lyssa virus, nipah virus
✓ fishes = capillariasis (by Capillaria
philippinensis), diphyllobothriasis (by

Diphyllobothrium latum >> fish tapeworm
infection) = both diseases is from eating
partially cooked fish
* diphyllobothriasis = asymptomatic in the first place
>> will lead to megaloblastic anemia
✓ rodents = leptospirosis ( by rats; also known as
Weir’s Disease; caused by a bacteria
spirochete = Leptospira interrogans),
hantavirus (shed in urine, feces, and saliva of
rodents >> matutuyo >> sasama sa air >>
magiinfect sa tao)
✓ canines = dogs (rabies)
✓ sheep, goat, cattle = anthrax (caused by
bacteria = Bacillus anthracis; contracted in fur/
wools for yarn)
▪ Zoonosis – infectious disease that is
transmissible under natural conditions from
vertebrate animals to humans; disease
transmitted from animals to humans
o Humans
– transmitted from person to person without
intermediaries (direct contact; no need for
another vehicle of transmission); classified as
carriers
– include the sexually transmitted diseases,
measles, mumps, streptococcal infection,
and many respiratory pathogens
▪ Carriers – person with in apparent infection
who is capable of transmitting the pathogen to
others; w/ no obvious signs and symptoms of
disease; they are harbouring the disease agent
but doesn’t exhibit signs and symptoms
o Types:
a) Incubatory Carriers – who have transmit
the agent during the incubation period before
clinical illness begins; shed the agent during
incubation period; w/o clinical manifestation
but are already harbouring the disease
agent; capable of infecting others before
onset of illness
examples: nasals, mumps, polio, pertussis
(whooping cough), influenza, diphtheria,
hepatitis
b) Convalescent Carriers – who have
recovered from their illness but remain
capable of transmitting to others; continue to
shed disease agent even during period of
recovery
examples: typhoid fever, amoebic or bacillary
dysentery, cholera, diphtheria, pertussis
(whooping cough)
c) Chronic Carriers – who continue to harbor a
pathogen such as hepatitis B virus or
Salmonella Typhi, the causative agent of
typhoid fever, for months or even years after
their initial infection; continue to shed
infectious agent for indefinite period of time

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– food, water, biologic products (serum, blood,
urine, plasma, organs), and fomites
– provides environment in which the agent
grows, multiplies, or produces toxin
examples: cooking and eating utensils,
clothing, toys, surgical instruments, water

(longer time: months or years)
examples: typhoid fever, hepa B/ Salmonella
Typhi, dysentery, cerebral spinal meningitis,
malaria, gonorrhea
*both incubatory and convalescent carriers are
considered as temporary carriers = bc they can shed
infectious agent in a short period of time; once
carrier/host has fully recovered, di na siya considered
as carrier kasi magaling na siya

c) Vector borne (mechanical or biologic)
– mosquitoes, fleas, and ticks
– carry an infectious agent through purely
mechanical means or may support growth or
changes in the agent
– living organism that can transmit infectious
agent between humans/from animals to
humans

– path by which a pathogen leaves its host and free
Portal of Exit

itself to external environment
– usually corresponds to the site where the pathogen
is localized (example: fecal or oral route)
*appropriate mode of transmission enables agent find
and enter another susceptible host
*skin = primary defense to pathologic agents

o

– favourable manner of organism to infect new host
❖ Direct
a) Direct Contact
– skin-to-skin contact, kissing, and sexual
intercourse
– contact with soil or vegetation harbouring
infectious organisms
– only possible if there is an opening/abrasion
in skin = if wala, transmission may be
challenging/impossible

Mode of
Transmission

– manner in which a pathogen enters a susceptible

host (oral route = ingestion, skin penetration =
inoculation, inhalation = respiratory tract, absorption
= mucus membrane)

– provide access to tissues in which the

pathogen can multiply or a toxin can act

– infectious agents use the same portal to enter a new

b) Droplet Spread
– spray with relatively large short-range
aerosols
– produced by sneezing, coughing, or even
talking
– by direct spray over a few feet, before the
droplets fall to the ground; short distances
examples: common colds, diphtheria, mumps,
pertussis (whooping cough), influenza

❖ Indirect
a) Airborne
– infectious agents are carried by dust or droplet
nuclei suspended and blown in air

– great distances
– airborne dust includes material that has settled
on surfaces and become resuspended by air
currents as well as infectious particles blown
from the soil by the wind
– droplet nuclei are dried residue of less than 5
microns in size and remain suspended in the
air for long periods of time and may be blown
over great distances
examples: Covid19, tuberculosis
b) Vehicle borne
– inanimate object/material (fomites) become
contaminated by infectious agent
– passively carry a pathogen

2 WAYS:
▪ Mechanical Transmission: spread of flies
and fleas of disease agent
▪ Biologic Transmission: causative agent
of undergoes maturation in an
intermediate host (needed by disease
agent to develop themselves asexually)
before it can be transmitted to humans

host that they used to exit the source host

– pathogens that cause gastroenteritis follow a socalled “fecal-oral” route because they exit the
source host in feces

Portal of
Entry

*susceptibility of a host to agents = depends on
quality of immune system, genetic or constitutional
factors, specific immunity, and nonspecific factors
that affect an individual’s ability to resist infection or
to limit pathogenicity of disease agent = done by
applying primary level of prevention and applicable
preventive measures
*genetic makeup may either increase or decrease
susceptibility
❖ Types of Immunity:
o Specific Immunity
– acquired/adaptive immunity
– slow to respond
– developed after body is exposed to specific
pathogen >> forms antigen
– has memory cells >> future protection if
pathogen is introduced again
•
2 CATEGORIES:
✓ humoral = antibody mediated (B-cells
produce antibodies)
✓ cellular = cell mediated (activated
antigen specific T-lymphocyte)

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2 SUBTYPE OF SPECIFC IMMUNITY:
✓ Active Immunity = natural; developed
due to production of antibody; ability of
body to produce own antibody
✓ Passive Immunity = artificial;
developed by antibody produced
outside, introduced inside; vaccines
and immunizations

*pub health centres reports cases to DOH >> DOH will collect data >> DOH
will make a study >> result is stabilized >> give feedback to pub health
professionals and public
*sources of data is needed to come up with mitigation and controls

o Non-specific Immunity
– innate/natural immunity
– first line of defense
– type of immunity we are born with
– provides generalized protection against
pathogen
– not confer memory cells against antigens = di
nila natatandaan ang foreign bodies entering
our bodies
examples: physical barriers (skin), mucus membrane,
antimicrobial enzymes of body (bodily fluids, tears,
saliva, stomach acid) reflexes (coughing/sneezing),
cells (macrophages. Basophils, natural killer T-cells)

a) Morbidity and Mortality reports – for local & state health dept;
submitted by healthcare providers, infection control practitioners,
or labs that are required to notify the health dept of any PX with a
reportable disease
b) Reports from investigations of individual cases and disease
clusters
c) Public disease registries
d) Health Surveys

•

❖ Sources of Information (these sources will create more room for
improvement of public health = bc these will pinpoint range & impact of
disease >> make course of action more specific and aligned in health
issues that has to be addressed immediately)

FIELD INVESTIGATION

CORE EPIDEMIOLOGICAL FUNCTIONS
I.
II.
III.
IV.
V.
VI.

Public Health Surveillance
Field Investigation
Analytic Studies
Evaluation
Linkages
Policy Development
PUBLIC HEALTH SURVEILLANCE

• Public Health Surveillance
– ongoing, systematic collection, analysis, interpretation, and
dissemination of health data to help guide public health decision
making and action
– give better and clear picture of health data that will help guide public
health decision making
– disease identification, cases investigation
PURPOSE: Portray the ongoing patterns of disease occurrence and
disease potential so that investigation, control, and prevention measures
can be applied efficiently and effectively; identify disease occurrences and
monitor patterns
Surveillance Cycle Illustration:

*NHIS (Natl. Health System Information System) = embodies by DOH

• Field Investigation
– actual performance of disease identification
– ongoing first actions that results from a surveillance case report or
report of a cluster
– investigation by the public health department
– if done appropriately >> will lead to learning more about natural
history, clinical spectrum, descriptive epidemiology, and risk factors of
the disease
– investigations often lead to:
a) identification of additional unreported or unrecognized ill persons
who might otherwise continue to spread infection to others
b) identification of a source or vehicle of infection that can be
controlled or eliminated
c) determination of how many other persons might have already
been exposed and how many continue to be at risk
d) learning more about the natural history, clinical spectrum,
descriptive epidemiology, and risk factors of the disease before
determining what disease intervention methods might be
appropriate

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ANALYTIC STUDIES
– used to define characteristics and variables involved in investigation of
public health issues
– once characteristic are defined >> expressed quantitatively/numerically >>
measuring incidence, prevalence, frequency, distribution, impact

association tests of significance,
confidence intervals, inferential
statistics is being conducted and
done
o Interpretation – putting the study
findings into perspective, identifying
the key take-home messages, and
making sound recommendations;
doing so requires that the
epidemiologist be knowledgeable
about the subject matter and the
strengths and weaknesses of the
study = for future reference for other
epidemiological study

❖ Types of Study Methods in Epidemiology
– Provides the Who What When and Where of
health-related events in a population

– Involves the study of disease incidence and
distribution by time, place, and person

– Calculation of rates and identification of

parts of the population at higher risk than
others

Descriptive
Epidemiology/Study

– Clusters or outbreaks of disease frequently
are investigated initially with descriptive
epidemiology

– When the association strength between

Epidemiology and Disease is strong, the
investigation may stop when descriptive
study is complete and control measures is
implemented immediately

PURPOSE: signify/validate association
between affected community

– Attempts to provide the Why and How of
health-related events in a population

– Seeks to provide predictions by using
valid comparison group/sampling

– Depicts entire methodology related to data
gathering and collected thru descriptive
studies

– What will happen in future or What are the
better things to do in the future

EVALUATION
• Evaluation
– done after decisions are made; recos are applied if programs are
success or failure
– ongoing process of determining, as systematically and objectively as
possible, the relevance, effectiveness, efficiency, and impact of
activities with respect to established goals.
o Effectiveness – ability of a program to produce the intended or
expected results in the field; goals are met
o Efficiency – ability of the program to produce the intended
results with a minimum expenditure of time and resources
❖ Types of Evaluation
a) Formative Evaluation: plans
b) Process Evaluation: operations
c) Summative Evaluation: impact or outcome (what are
things that are met and achieved)

– Test the hypothesis generated by
descriptive studies

Analytical
Epidemiology/Study

❖ Aspects
o Design – determining the
appropriate research strategy and
study design, writing justifications
and protocols, calculating sample
sizes, deciding on criteria for
subject, choosing an appropriate
comparison group, and designing
questionnaires.
o Conduct – securing appropriate
clearances and approvals, adhering
to appropriate ethical principles,
abstracting records, tracking down
and interviewing subjects, collecting
and handling specimens, and
managing the data
o Analysis – describing the
characteristics of subjects;
calculation of rates, creation of
comparative tables, and
computation of measures of

LINKAGES

– epidemiologist usually participates as either a member or the leader of a
multidisciplinary team

– strengthen relationship of health professionals working collaboratively to
strengthen public health networks

– to promote current and future collaboration, the epidemiologists need
to maintain relationships with staff of other agencies and institutions

– include official memoranda of understanding, sharing of published or
on-line information for public health audiences and outside partners,
and informal networking that takes place at professional meetings
POLICY DEVELOPMENT

– epidemiologists provide input, testimony, and recommendations

regarding disease control strategies, reportable disease regulations, and
health-care policy

– necessitate adherence to standard health protocols, regulations on how
protocols should be followed and implemented

– “guiding star” to implement of standard health protocol

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