Med

1. Introduction to Epidemiology
Of Communicable Diseases
Prof. Dr. Nadia Montasser

2. ILOs:
 Definitions of health, , epidemiology
Public health and community medicine
definitions, essential functions
 Health dimensions reflected by definitions of health
 Spectrum of health from perfect health to death
 Determinants of health :Host factors ,
Environmental factors and Agent
 Risk factors
 Chain of infection

3. PUBLIC HEALTH:
 Public health is "the art and science of
promoting health, preventing disease and
increasing the span of healthy life through
organized efforts of the society".
 Another definition, which has almost the
same, meaning "public health can be defined
as the combination of sciences, skills and
beliefs that are directed to the maintenance
and improvement of the health of all the
people."

4. Essential Public Health
Functions:
 Health education
 Prevention and control of communicable diseases
 Care for special groups as mothers, children, elderly
and occupational workers
 Assessment of health needs, plans and supports the
provision of health care services to the population.
 Protection of the environment
 Health legislation and health regulations

5. Community medicine
Community medicine is the application of
the principles of public health to
communities.
A community is a group of individuals
sharing an identity, culture and operates
through common institutions and
organization.

6. Comparison between clinical
medicine and community medicine
Difference Clinical medicine Community medicine
Objective
Information
Investigations
Diagnosis
Resources
Management
Evaluation
Patient cure
Complaint, history
Laboratory tests, X-ray, other tests
Differential diagnosis
Available therapy
Treatment
Follow-up of patient
Health improvement
Morbidity, mortality and related factors
Survey studies
Community diagnosis &
Priority setting
Health care facilities & services
Assessment of health status

7. Epidemiology
Epidemiology is the science of the
frequency, distribution, and determinants
of disease among specific populations at
a certain place and time.
 It applies methods and tools to gather the
information needed to understand health
problems and disease processes in order to
identify effective preventive and control
measures.

8. Definition Of HEALTH
 The world Health Organization (WHO) defines
health as “a state of complete physical,
mental and social well being and not merely
the absence of disease or infirmity."
 This is ideal health, and no one could possibly
be complete from all mentioned aspects.
However, the definition provides a goal
towards which we should be working to
continuously try to improve our health.

9. September 20, 2
9
"A state of complete physical, mental & social well
being and not merely absence of disease or
infirmity"
Physical health Mental health Social health
All body systems
are properly
functioning
without any
disease.
A balance
between individual
& his environment
with ability to:
1. Adjust within
himself & others.
2. Cope with stress.
Ability of person to:
1. Live & react in
harmony.
2. Adjust within family
& community.
3. Be an active member
of a community.

10. SPECTRUM OF HEALTH:
 1. Ideal &Positive health:
This conforms to the WHO definition of health with perfect
continuing adjustment between the individual and the
environment.
 2 . Negative (marginal) health:
 Freedom from illness & became ill on the slightest adverse
stimulus.
3. Inapparent disease (pre-clinical):
Inapparent disease is not recognized by the individual, but
can be discovered by examination and screening tests.
 4. Apparent disease:
The individual is aware that he is suffering from illness
whether he seeked medical care or not
 5. Death: The end of the health spectrum.

11. Disease
Abnormal state in which the body
is not capable of responding to
or carrying on its normally
required functions.

12. (Forms of Disease):
Acute: illnesses generally develop suddenly and
last a short time relatively severe, of short
duration and often treatable.
 Usually the patient either recovers or dies.
 Acute conditions are often caused by a virus or an infection, but
can also be caused by an injury resulting from a fall or an
automobile accident, or by the misuse of drugs or medications.
 E.g. acute Asthma attack, bone fracture , Meningitis , Heart attack,

13. Subacute:
 Intermediate in severity and duration,
having some acute aspects to the
disease , but of longer duration and with
a degree of severity that detracts from a
complete state of health.

14. Chronic: Conditions develop slowly and may
worsen over an extended period of time, Less severe
but of long and continuous duration, lasting over a
long time periods or even long life.
 Patient may not fully recover and the disease can get worse
overtime
 Chronic conditions are often caused by unhealthy behaviors that
increase the risk of disease—poor nutrition, inadequate physical
activity, overuse of alcohol, or smoking.
 Social, emotional, environmental, and genetic factors also play a

15. DETERMINANTS OF HEALTH
Environment
Host factors Causative Agent

16. Host Factors:
The individual susceptibility or resistance to disease depends on several factors
including:
 Heredity and genetic factors: chromosomal
anomalies, errors of metabolism, mental retardation
and certain tumors
 Age and sex:
 Acute and infectious diseases are usually more
common among the young population.
 Chronic diseases are more common among the aging
populations.
 Certain diseases may be more common among males
or among females.
 Coronary heart disease is uncommon among females
in the reproductive age.
 Gall bladder stones are common among fatty fertile
females of forty

17. Host Factors:
…………………..cont
 Nutritional status
 Nutritional status of the individual
determines ability to resist infectious diseases.
 Malnutrition interacts with infection.
 Malnutrition may activate a dormant
tuberculosis infection
 Under nutrition leads to deficiency diseases,
while over nutrition leads to overweight and
obesity.

18. Host Factors: ………….. cont
Immunity
 Natural immunity as intact skin and mucous membranes protects against
several infections
 Clinical or subclinical infection may produce immunity which gives the
person the ability to resist the development of disease if future infection takes
place.
 Some diseases produce solid immunity for life.
Physiologic state: Pregnancy, lactation and growing children
(vulnerable groups) are undergoing physiologic changes which make them
more susceptible to some health hazards.
Life style: Life style is the way people live. It includes their behavior
and beliefs, cultural values, activities and personal habits e.g. smoking.
Socio-economic status: The health status is affected by
the education, occupation and economic condition of the people.

19. Environmental Factors:
The environment is the medium which
surrounds us.
The components of the environment are:
 Physical Environment: the non-living things of the
external environment as air and water.
 Biological Environment : the living components of
the external environment namely insects, animals
and microbes
 Social Environment: Comprises human culture, the
social norms, relations and behaviors

20. Causative Agent:
Biologic agents: These are the causes of
communicable diseases; they include
viruses, bacteria, parasites, and fungi.
Nutritional agents: Deficiency of nutrients
cause diseases; e. g., iron deficiency
anaemia, vitamin deficiency diseases
Chemical agents: These may enter the body
through different routes as ingestion,
inhalation and sometimes through the skin.
Examples are poisons, insecticides, and
even some drugs.

21. Causative Agent:…………………
cont.
 Physical agents: Excessive heat or cold,
electricity, vibration and radiation,
 Mechanical agents: causing injuries or
accidents.
 Metabolic agents: e.g., phenylketonuria,
galactosemia,..
 Genetic agents: as in Down's syndrome.
 Functional: Dysfunction of certain organs or
gland may produce disease, e.g., Diabetes
Mellitus.

22. Risk Factors:
 These are factors that make an individual, a family,
a group of individuals, or a community, more prone
to a specific disease due to the presence of a
certain factor which is not the direct causative
agent.
 The causative agent may be known or unknown.
 The risk factors are related to the host or
environment.

23. Diseases Classification
Diseases can be classified into :
1. Communicable diseases : Disease result from infection
that transmitted to human directly or indirectly .
2. Non- Communicable diseases : Impairment in body
function or structure associated with exposure to risk
factors

24. Spectrum of disease
 (Inapparent, mild, moderate, severe or fatal)
 Class A: Inapparent infection
 Frequent inaaperent infections (Low pathogenicity and
low virulence)
 Examples: Tuberculosis, Polio, Hepatitis A, Meningitis, AIDS
 Class B: Classic cases
Frequent clinical disease and few deaths (High
pathogenicity & low virulence) ,Examples: Measles,
Chickenpox
 Class C: Severe or Fatal infections
 Infections usually fatal (High pathogenicity & high
virulence) ,Examples: Rabies, Hemorrhagic fevers

25. Patterns of occurrence of disease in
communities
Sporadic : Scattered cases which are
separated from each other. There is unknown
common source of infection as polio, herpes
zoster and meningococcal meningitis.
Endemic:The constant presence of a disease
in a geographic area or the presence of an
illness or disease in a certain area all over the
year. e.g. bilharziasis .
Outbreak: A more or less localized epidemic
affecting large number of a group, in the
community e.g. outbreak of food poisoning

26. Patterns of occurrence of disease in
communities
 Epidemic: The sudden appearance of an illness
or disease in certain area and in a specific time
OR it is the occurrence of an illness in excess of
normal expectation based on past experience.
 Epidemico-Endemic: It is an epidemic upon
endemic disease.
 Pandemic: The appearance of a disease in an
epidemic form spreading from one country to
another in a short time or occurs at the same
time in different countries, as influenza and
cholera. (Epidemic in more than one country).

27. Chain of infection

28. Development &
Maintenance
of any inf. dis. in the
community
A chain of factors necessary
for:
Chain of infection (infectious cycle)

29. AGEENT IN
RESERVOIROR
SOURCE
SUSCEPTIBLE
POPULATION
MODE OF TRANSMISSION

30. Chain of Infection
Pathogen
- is the disease causing agent

31. 1- Infectivity
2- Pathogenicity
3- Virulence
4- Antigenicity
5- Resistance outside the host.
Agent factors that affect disease transmission:
(2) Causative agent
Agents causing infectious diseases.
(viruses, bacteria, protozoa, parasites, fungi).

32. 100
x
es
susceptibl
of
Number
cases
2ry
of
Number
rate
attack
2ry 
It is measured by 2ry Attack Rate
The proportion of exposed susceptible persons who become
infected.
Ex; - High infectivity: Measles, Polio
- Low infectivity: Leprosy
Ability of agent to invade & multiply (produce inf.) in a
susceptible host.
1- Infectivity:

33.  The ability of the organism to produce specific
clinical reaction after infection
 It is measured by:
2- Pathogenicity:
cases
l
Subclinica
cases
Clinical
atio of clinical to subclinical case =

34.  Ability of organisms to produce severe pathological
reaction.
 It is the proportion of persons with clinical disease who
become severely ill or die.
 It can be measured by case fatality rate.
3- Virulence:
100
disease
that
of
cases
of
number
Total
disease
a
from
deaths
of
number
Total
rate
fatality
Case x


35.  Ability of organism to produce specific immunity
(antibodies)
 It can be measured by:
The 2nd
attack (Re-infection):
- rare in measles,
- occurs in influenza.
4- Antigenicity
(Immunogenicity):

36.  The ability of the agent to survive adverse
environmental conditions.
Examples environmental
resistance
• Influenza virus Low (fragile)
• mycobacterium TB. High
5- Resistance:

37. Chain of Infection
 is the habitat in which an infectious agent normally
lives & grows
 Human: cases, carriers
 Animal: called zoonoses
 Environmental: plants, soil, and water
Pathogen
Reservoir

38. Human reservoir
1)Cases
 Typical cases: Persons with typical specific manifestations of
the disease.
 Atypical cases: With atypical manifestations (difficult to be
diagnosed).
- 2)Carrier Person with unapparent infection (no symptoms) but
is capable of transmitting the pathogen to others discharging
the organism).
September 20, 2025
38

39. Carriers are dangerous because:
1. Their number is much larger than cases.
2. No clinical manifestation& not diagnosed:
 - No precautions taken by them or by contacts.
 - They carry normal life, move & spread infection.
1. In some diseases, Carriers are:
 - Dangerous, e.g. food handlers & school personnel.
 - Infective for long period (even for life) e.g. carrier of typhoid,
hepatitis B
September 20, 2025
39

40. According to
1) Relation to the case
2) Duration of carrier state
3) Foci of infection, Discharges that carry organism
outside body
4) Flow of organisms outside body. Continuous or
intermittent
Classification of carriers:
The following diseases have no carriers:
- Influenza - Measles - Herpes Zoster - Whooping Cough -
T.B.

41. (1) According to relation to the case:
Infective during lP, e.g. Cholera,
Hepatitis A
Incubatory carrier
Infective during recovery, e.g. Cholera ,
Typhoid,
(clinical but not bacteriological
recovery)
Convalescent carrier
Contact with a case or polluted
environment (contaminated food, water)
Contact (healthy)
carrier
Infective for days (last days of I.P)
cholera
Transient carrier
Infective for few weeks up to few months
(viral B hepatitis 3 ws up to 3 ms)
Temporary carrier
Infective for years, Typhoid, Hepatitis
B
Chronic carrier
(2) According to duration of carrier
state:

42. Respiratory GIT Urinary
Throat & nose:
• Diphtheria,
• Staph. Aureus
Nasopharynx:
• Meningococci
• Pneumococci
Small intestine:
• S. typhi
• S. paratyphi
Large intestine:
• amaebiasis
S. typhi
paratyphi
(3) According to foci of infection and discharge that
carry organism outside the body
Skin lesion
Respiratory
discharges
Faecal Urine Skin
discharges
•Staph. aureus
in food
poisoning
•Skin diseases

43. 4) Flow of organisms outside body.
Continuous or intermittent

44. Chain of Infection
 is the path by which an agent leaves the
source host
Pathogen
Reservoir
Portal
of exit

45. Portal of exit:
It is the path by which an agent leaves the source
host:
September 20, 2025
45
Respiratory tract: Sneezing, coughing Influenza, measles,
COVID-19
GIT: - Vomitus Cholera
- Stool Typhoid
Genitourinary
tract:
- Urine Typhoid
- Genital discharges Gonorrhea, syphilis
Skin & mms discharges: - Infected wounds Staph. aureus
- Skin eruption Chicken pox
- Eye discharge Infective conjunctivitis
Blood: - Contaminated syringes
- Blood transfusion
HIV, viral hepatitis

46. 3-Period of infectivity
Dis. having
no carriers
Dis. having
carriers
Infectivity ends by
convalescence
Infectivity extends
After convalescence
infectivity ends by clinical cure
temporary
carriers
chronic
carrier
Infectivity usually start the last days or weeks of IP
viral hepatitis B, C,
typhoid
e.g., measles,
chicken pox

47. Chain of Infection
Modes of Transmission
Direct
- Direct contact - blood transfusion
- Droplet spread -transplacental
Indirect
- Airborne
- Vehicleborne
- Vectorborne
Pathogen
- how pathogens
are passed
Reservoir
Portal
of exit Trans-
mission

48. Chain of Infection
Pathogen
Reservoir
Portal
of exit
Trans-
mission
- agent enters
susceptible host
Portal
of entry
Respiratory
Oral
Skin
Intravenous
Gastrointestinal

49. Chain of Infection
Pathogen
Reservoir
Portal
of exit
Trans-
mission
- Final link is
a susceptible host
Portal
of entry
New
Host

50. Susceptible Host
-the new Host that
accepts the pathogen
The support of pathogen life
& its reproduction depend
on the degree of the host’s
resistance.

51. Human with strong
immune systems are
better able to fend off
pathogens.

52. Human with weakened
immune systems are more
vulnerable to the support &
reproduction of
pathogens.