Epidemiology of Disease and Health Condi3ons CLS 351

Epidemiology of disease and
health condi3ons
CLS 351
Outline
• Disease classifica*on.
• Epidemiologic transi*on.
• Describing natural history and spectrum of disease.
• Describing the epidemiologic triangle (Epidemiological triad).

• Disease occurrence/outbreak.
• Define epidemic, pandemic and endemic diseases.

Classification of diseases and health problems
1. Organ or system:
• kidney diseases,
• Respiratory infec*ons
2. Causa3ve agents:
• Biological: viruses, bacteria,…etc.
• Chemical: drugs, pes*cides, …etc.
3. Dura3on:
• Acute or Chronic.
4. Communicability:
• Communicable (infec*ous)
• Non-communicable (non-infec*ous)
Acute vs chronic diseases
Acute conditions are severe and sudden in onset.
• This could describe anything from a broken

bone to an asthma aIack.
A chronic condition, by contrast is a long-

developing syndrome, such as osteoporosis or
asthma.
• Note that osteoporosis, a chronic condi*on,
may cause a broken bone, an acute condi*on.
• An acute asthma aIack occurs in the midst of
the chronic disease of asthma.
• Acute condi*ons, such as a first asthma
aIack, may lead to a chronic syndrome if
untreated.
Communicable vs non- communicable diseases
Communicable (infectious) diseases

• Caused by biological agents or
their products.
• Transmissible from one individual
to another.
Non-communicable diseases
• Not caused by infec*ous agents
• Not transmiIed from one person
to another.
Infectious diseases
Infec3ous (communicable, transmissible) disease

• caused by an infec*ous agent.
Infec3on
• occurs when the infec*ous agent enters the body.
• begins to reproduce in the body.
• may or may not lead to a disease.
Not all infec,ons produce clinical diseases.
Non infectious diseases
Non-communicable (noninfectious) diseases:

Because of their chronic nature, they can deplete a
community’s resources.
Many of them are called multi-causation diseases
e.g. Many factors contribute to diabetes such as
• Genetics
• Diet
• Obesity
Classify the following diseases as infections or non-
infectious:
 AIDS
 Tuberculosis
 Cholera
 High blood pressure
 Heart disease
 Pneumonia
 Cancer
Epidemiologic transition
• De;ni3on: In demography and medical geography, epidemiological

transi3on is a theory which “describes changing popula*on paIerns in
terms of fer*lity, life expectancy, mortality, and leading causes of death”.
• Such transi*ons are important because they tell us a great deal about
the true founda*ons of diseases: what really causes the disease.
• Throughout the past 100 years, a major epidemiologic

transi*on has occurred.
Epidemiologic transition and the global burden of disease
During the epidemiologic transi*on, a long-term shiV occurs in mortality and

disease paIerns whereby pandemics of infec*on are replaced by degenera*ve
and man-made diseases.
Leading cause of deaths worldwide
Murray, C. J., Vos, T., Lozano, R., Naghavi, M., Flaxman, A. D., Michaud, C., ... & Aboyans, V. (2012). Disability-adjusted life years (DALYs)
for 291 diseases and injuries in 21 regions, 1990–2010: a systematic analysis for the Global Burden of Disease Study 2010. The
lancet, 380(9859), 2197-2223
Global risk factor ranks for all ages and sexes combined
in 1990, and 2010, and percentage change
(Lim et al. 2012)

Natural History of Disease:
Describes what happens from the beginning to the

end of disease without interven*on.
Death
Good Subclinical Clinical

Changes
Health Changes
Recover
y
Definition of the natural history of diseases:
“Progress of a disease process in an individual over *me star*ng
from the moment of exposure to the causal agents un*l major
possible outcomes occur”.
Stages of the natural history of diseases:
1-Stage of pre-pathogenesis (susceptibility) = well:

The disease has not developed, but the human host
is very much exposed to risk factors which increase
the likelihood that the disease will develop.
Stages (Cont.):
2 Stage of pathogenesis:
It starts with the exposure to the causes or risk factors of a
disease, and the start of pathologic changes (two stages):
1. pre-symptoma*c stage (sub-clinical stage): no signs or
symptoms.
2. clinical disease stage: there are sufficient anatomic
and/or func*onal changes have occurred (signs and /or
symptoms).
Exposure:
• Microorganism in infec*ous diseases.
• Factor in non-communicable diseases (e.g. tobacco
smoke in lung cancer).
Stages
(Cont.):
3- Outcome (end results) of disease
process:
• Recovery.
• Chronicity.
• Disability (any limita*ons of person’s ac*vi*es mental
or physical as a result of acute or chronic condi*on).
• Death.
Health – disease spectrum
On the health-disease spectrum

– The lowest point is death
– the highest point is positive health.
There are degrees or "levels of health", as there are
degrees or severity of illness.
22
Epidemiological triangle
(Epidemiological triad)
• Diseases do not occur randomly in a

popula*on, more likely to occur in some
members than others.
• According to this theory, exposure to

an agent does not necessarily lead
to disease.
• It is believed that disease is the result of

an interac*on between agent, host and
the environment.
Epidemiologic Triangle of Infection
Epidemiological Triangle (non-infectious)
Agent: is the direct cause of disease without which a

specific disease cannot occur.
Presence agent alone is not sufficient to cause disease.
So, an agent could be:
– Biologic (microorganisms/bacteria/virus)
– Chemical ( toxins, drugs, tobacco)
– Physical (fire, radia*on, trauma)
– Nutritional (lack of, excess)
Host: refers to human or non-human and specifically the

par*cular person or group of people suscep*ble to illness.
Host factors are: age, sex, marital status, race,

immunity, occupa*on, educa*on, religion, culture,
lifestyle, health status.
• Different hosts may have different reac*ons to the

same agent.
• All the host factors are called “intrinsic factors”.
Environment: is the medium where people live,

environment refers to external condi*ons that
cause/allow disease transmission.
- All the environmental factors are called “extrinsic
factors”.
 Weather.
 Popula*on density.
 Geography.
 Season of the year.
Environment is classified into:

a. Physical environment
Weather, light, air quality, radia*on, pressure.
b. Biologic environment
Mosquitoes and flies.
c. Socio-economic environment
Overcrowding and poverty.
• To be effec*ve, we need to interrupt

epidemiological triangle at any point.
• Preven*on or controlling spread of diseases through
understanding:
– Causes of diseases.
– Groups likely to get diseases.
– Geographical factors conducive to spread.
Disease occurrence
• The amount of a par*cular disease that is usually present in

a community is referred to as the baseline or endemic level
of the disease.
• This level is not necessarily the desired level, but rather is
the observed level.
• While some diseases are so rare in a given popula*on that
a single case warrants an epidemiologic inves*ga*on (e.g.
Rabies, Plague, Polio), other diseases occur more
commonly so that only devia*ons from the norm warrant
inves*ga*on.
• Disease occurrence can be: Epidemic, Endemic, Pandemic

and Sporadic.
(Gordis
2014)
For each of the following situations, identify whether it reflects:
A. Sporadic disease
B. Endemic disease
C. Pandemic disease
D. Epidemic disease
22 cases of legionellosis occurred within 3 weeks among residents of

a par*cular neighborhood (usually 0 or 1 per year).
Over 20 million people worldwide died from influenza in 1918–

1919.
Single case of histoplasmosis was diagnosed in a

community.
About 60 cases of gonorrhea are usually reported in this region per

week, slightly less than the na*onal average.
Epidemics
Epidemics occur when an agent and suscep*ble hosts are
present in adequate numbers, and the agent can be effec*vely
conveyed (transmiIed) from a source to the suscep*ble hosts.
 Epidemic Patterns
Epidemics can be classified
according to their manner of
spread through a popula*on:
• Common-source/Point.
• Propagated
Common-source/Point
• A common-source outbreak is one in which a group of persons

are all exposed to an infec*ous agent or a toxin from the same
source.
• If the group is exposed over a rela*vely brief period, so that

everyone who becomes ill does so within one incuba*on period,
then the common-source outbreak is further classified as a
point-source outbreak.
• E.g.
• The epidemic of leukemia cases in Hiroshima following the atomic bomb
blast.
• The epidemic of hepa**s A among patrons of the Pennsylvania restaurant
who ate green onions each had a point source of exposure.
Example of point-source
outbreak
Source: Centers for Disease Control and Preven*on. Unpublished data; 1979.
Propagated
A propagated outbreak results from transmission from one person

to another.
 Usually, transmission is by direct person-to-person contact, as
with syphilis.
 Transmission may also be vehicle-borne (e.g., transmission of
hepa**s B or HIV by sharing needles) or vector-borne (e.g.
transmission of yellow fever by mosquitoes).
 In propagated outbreaks, cases occur over more than one
incubation period.
 The epidemic usually wanes over a few genera*ons, either
because the number of suscep*ble persons falls below some
cri*cal level required to sustain transmission, or because
interven*on measures become effec*ve.
Example of propagated
For each of the following situations, identify the type
of epidemic spread with which it is most consistent.
A. Point source
B. Propagated
21 cases of shigellosis among children and workers at a

day-care center over a period of 6 weeks, no external source
iden*fied (incuba*on period for shigellosis is usually 1—3
days).
43 cases of norovirus infec*on over 2 days traced to

the ice machine on a cruise ship (incuba*on period for
norovirus is usually 24–48 hours).
 In a common source epidemic, subjects are exposed to a
common noxious influence. If the group is exposed over a
rela*vely short period then disease cases will emerge over
one incuba*on period.
 A propagated epidemic occurs when a case of disease

serves as a source of infec*on for subsequent cases and
those subsequent cases, in turn, serve as sources for later
cases. In theory, the epidemic curve of a propagated
epidemic has a successive series of peaks reflec*ng
increasing numbers of cases in each genera*on.
End of Lect-2

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Epidemiology of disease and

• Describing natural history and spectrum of disease.

• Describing the epidemiologic triangle (Epidemiological triad).

• Deﬁne epidemic, pandemic and endemic diseases.

Acute conditions are severe and sudden in onset.

• This could describe anything from a broken

A chronic condition, by contrast is a long-

Communicable (infectious) diseases

Infec3ous (communicable, transmissible) disease

Non-communicable (noninfectious) diseases:

• De;ni3on: In demography and medical geography, epidemiological

• Throughout the past 100 years, a major epidemiologic

During the epidemiologic transi*on, a long-term shiV occurs in mortality and

(Lim et al. 2012)

Describes what happens from the beginning to the

Good Subclinical Clinical

1-Stage of pre-pathogenesis (susceptibility) = well:

On the health-disease spectrum

• Diseases do not occur randomly in a

• According to this theory, exposure to

• It is believed that disease is the result of

Agent: is the direct cause of disease without which a

Host: refers to human or non-human and speciﬁcally the

Host factors are: age, sex, marital status, race,

• Diﬀerent hosts may have diﬀerent reac*ons to the

Environment: is the medium where people live,

Environment is classiﬁed into:

• To be eﬀec*ve, we need to interrupt

• The amount of a par*cular disease that is usually present in

• Disease occurrence can be: Epidemic, Endemic, Pandemic

22 cases of legionellosis occurred within 3 weeks among residents of

Over 20 million people worldwide died from inﬂuenza in 1918–

Single case of histoplasmosis was diagnosed in a

About 60 cases of gonorrhea are usually reported in this region per

• A common-source outbreak is one in which a group of persons

• If the group is exposed over a rela*vely brief period, so that

A propagated outbreak results from transmission from one person

21 cases of shigellosis among children and workers at a

43 cases of norovirus infec*on over 2 days traced to

 A propagated epidemic occurs when a case of disease

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