Disease is the result of forces within a dynamic system called an “epidemiological triad” which consisting of: agent, host and environment.

1. Dr. Dalia El-Shafei
Assistant professor, Community Medicine Department,
Zagazig University

2. 1) To recognize the infectious cycle (epidemiological triad)
2) To understand the chain of infection.
3) To understand the natural history of the disease

3. • Disease is the result of
forces within a dynamic
system called an
“epidemiologic triad”
which consisting of:

4. • To put preventive & control measures, limit their
spread in the community and limit their complications
in cases
Communicable diseases: are diseases transmitted from
one case to another. There is a cycle for transmission of
infection from one host to another.

7. Biological
agent
Parasites
Protozoa
“Amoebae”
Metazoa
“Ascaris”
Fungi “Tinea” Bacteria“TB”
Rickettsia
“Typhus”
Viruses
“AIDs”

8. Natural
characteristic of
the agent
• Production, Metabolism, Motility, Production of toxin.
Characteristic of
agent related to
infection in
human
• Infectivity: Ability of an organism to invade & infect a
host.
• Pathogenicity: Ability to cause disease.
• Virulence: Ability to cause serious complications /death.
• Antigenicity: Ability to stimulate host production of
antibody

10. Source of infection
Immediate person
or object from
which agent passes
to host.
Reservoir
Natural habitat, in
which an agent
lives, grows &
multiplies
Human reservoir
Animal reservoir
Non-living things
reservoir
Source of infection may be a part of the reservoir
or not “food poisoning (reservoir is the carrier
who contaminate the food, while source of
infection is the food itself)”.

11. Cases: individuals who are suffering from the disease.
Typical: showing typical manifestation of the disease & their level
of severity may range from being mild to moderate to severe
Atypical: does not show the typical manifestation of diseases or
Subclinical / in apparent: can pass undiagnosed.
Carriers: a person who is harboring the organism (allows its
multiplication inside his body) without showing signs &
symptoms. And is capable of transmitting the disease to other
person.
Carriers “the most dangerous source of infection”
Subclinical or in apparent infection Atypical cases
Typical cases.

12. Typesofcarriers
Timing of
transmitting agent
Incubatory
Convalescent
Contact
Duration
Temporary
Chronic
Portal of exit
Respiratory
Fecal
Urinary

14. • Diseases that can be transmitted under natural conditions from
vertebrate animals to humans are called “Zoonosis” (e.g.
rabies, yellow fever, plague, anthrax, brucellosis).
• Famous animal reservoirs: cat, dog, horse, cattle, poultry and
rodents.

15. • Soil can also act as reservoir of infection (e.g. soil may
harbor agents that cause tetanus & anthrax).
• Water.
• Food: meat, milk, vegetables.

16. Path by which an agent leaves its human or animal source host
Respiratory tract e.g. influenza virus
Genitourinary tract e.g. sexually transmittal diseases
Alimentary tract e.g. hepatitis A virus (HAV)
Blood e.g. hepatitis B virus (HBV)
In-utro transmission e.g. rubella, CMV

18. • Directly: droplets
“kissing, sneezing,
cough, spitting” (short
distance transmission).
• Indirectly: droplet
nuclei or contaminated
articles.
Droplet or air
borne
• Directly: water,
hands, food
• Indirectly: flies,
cockroach, dust
Food borne
• Intrauterine infection
as German measles,
malaria, syphilis,
AIDS.
Mother to
infant during
pregnancy
• Direct contact with case or infected
animal.
• Indirect: clothes, fomites, dust, water
pool. Infection by contaminated
syringes occurs through skin
penetration.
Contact method “Skin
& mucous membrane”
• Biological transmission: development
of new stages of organism occur inside
vector “malaria”.
• Mechanical: vector just carry
organism from case to new host
“typhoid”.
Arthropod borne

19. • Route the agent uses to get into the new host.
• In general, portal of entry is similar to portal of exit
Respiratory
tract
Ingestion Dermal
Blood borne
Mucous
membranes

20. Intrinsic factors that influence an individual's exposure,
susceptibility, or response to a causative agent
Host
Immunity
Virulence
of
Organisms

21. Age Sex Pregnancy Nutrition
Trauma,
stress, fatigue
Occupation Environment
Education &
culture
Socio-
economic
standard
Vaccination
Factors affecting host immunity

22. Herd immunity (immunity of the community)
Describes the immunity level that is present in a population
group.
Provides an immunological barrier to the spread of diseases
in the community.
May be acquired after frequent mass vaccinations
The higher the herd immunity the higher the power to
defense of an epidemic occurrence in the community.

24. Types of immunity
Non-
specific
• Mechanisms that prevent entrance of agents to the body
• Intact skin
• Cough, sneezing reflexes
• Gastric juices
• Normal bacterial flora
• Secretions: saliva, tears.
Specific
• Cell-mediated immunity: Delayed hypersensitivity &
regulation of anti-body production
• Humeral immunity: Circulating antibodies inactivate
specific antigens or organisms & prevent replication in body.
Antibodies are disease-specific.
• Naturally acquired “Active or Passive”
• Artificially acquired “Active or Passive”

26. Active Passive (maternal immunity)
After infection, the
body forms antibodies
against that infection.
- Solid immunity:
measles
- Short time
immunity: influenza.
Antibodies from mothers during pregnancy
are transmitted to infant.
It depends on diseases the mother had
exposed to before pregnancy to form
antibodies.
No cellular immunity is transmitted because
of placental barriers.
Antibodies also are present in colostrum
breast milk secreted on the 1st days after
labor.
Naturally Acquired Immunity

27. Artificial acquired active immunity
• Immunity is induced by immunization by vaccine
which stimulates the body to form immunoglobulin.
• Duration of immunity depends on the type of vaccine.
Live vaccine (small pox)
& live attenuated vaccine
(MMR)
• Give lifelong immunity.
Killed vaccine
• Contains killed agents
but still retain their
antigenicity as typhoid,
• Give short time or low
level of immunity.

28. Whole organism “polio & measles”
Capsule of organism “meningitis”
Surface antigen of virus “HBV”
Toxin secreted by organism
“diphtheria & tetanus”
Vaccine preparation

30. Intradermal “BCG” Oral “Polio”
Subcutaneous or
Intramuscular “MMR”
Intranasal
“Influenza”
Route of Vaccination

33. • Spoiled vaccine: change in its optimal temperature (most
vaccines need cold chain in transportation & storage)
• Vaccination during the first 6 months of life: maternal
immunity is still present & interferes with action of
vaccine.
Causes of failure of Active Immunization

34. Seroprophylaxis:
Ready-made “Ig” given for rapid protection after
exposure to infection for prevention or attenuation of
disease severity.
The body has no role in immunity.
It usually gives short duration of immunity.
Ig or Antitoxins:
• Animal origin as tetanus, diphtheria antitoxins
• Human origin: from plasma of actively immunized
persons as rabies serum.
Artificial acquired passive immunity

35. Chemoprophylaxis:
Administration of antimicrobial drug before exposure or just
after exposure to prevent occurrence of disease (not for
treatment).
May be the basic preventive measure: Penicillin for rheumatic
fever, tetracyclines for cholera, INH for TB, rifampicin for
meningitis.
Disadvantages:
• Cost: expensive compared to expected potential benefit”
• Side effects of drugs
• Development of drug resistant strains
• Temporary protection.
Artificial acquired passive immunity

36. • The time interval between contact with an agent and the
1st clinical evidence of the disease.
• It depends on:
Portal of entry (defense mechanism).
Ability of multiplication (infectivity).
Number of agents.
Level of antibody in the host.

37. The IP varies individually according to:
Defense mechanism “Ability to react against agent
invasion in the body”.
External defense mechanism
• Cough & Sneeze reflex
• Vomiting & Gastric acid
• Tears
• Skin
Internal defense mechanism
• Cellular & humeral immunity.
• If external defense mechanism
can’t eliminate agent; internal
defense mechanism continue
process by:
• Inflammation
• Isolation by fibrocytes
• Macrophage phagocytosis
• Antibody reaction

38. Importance of IP:
• Tracing the source of infection & contact
• Period of surveillance
• Immunization
• Identification of point source or propagated epidemics
• Prognosis e.g. in rabies & tetanus, the shorter the IP is, the
worse the prognosis of disease.

39. Differs from disease to another & from one case to another.

41. Sporadic
cases
Outbreak Epidemic Endemic Pandemic Epizootic

42. Sporadic
cases
• Infected cases have no common source of infection.
• Spread all over far away areas not related to each other.
Outbreak
• Spread of infection in confined place as school or camp.
• Usually there is common source for infection, if
controlled the incidence of infection decreases.
Epidemic
• Widespread of infection, unexpected increase in No. of
already present infection in certain area in short period.
• Or it is introduction of new infection to the country
never shown that disease before as SARS, Avian flu

43. Endemic
• Constant presence of disease within a
given geographic area or population
group.
Pandemic
• Spread of infection all over the world or
in many countries e.g. avian flu, Aids.
Epizootic
• Spread of Zoonotic disease in large scale
among animals e.g. cow mad disease.

45. Common source epidemic
• All cases diagnosed are
infected from a common
source at certain point of
time.
• The resultant cases all
develop with one IP of
disease e.g. epidemic of
food poisoning.
Propagated source epidemic
• Cases are exposed to more
than one source of infection.
• The epidemic usually shows
a gradual rise & falls over a
longer period of time e.g.
epidemic of HAV.

46. Set of procedures used to identify the cause responsible for the
disease, the people affected, the circumstances & mode of
spread of the disease, and other relevant factors involved in
propagating the epidemic Control disease spread.
Especially important if the epidemic:
It is a challenging task for health workers.

47. • Verify the diagnosis or causes.
• Establish the existence of an epidemic.
• Description of epidemic as regard time, place & person “TPP”.
• Develop hypotheses to explain the occurrence of the epidemic.
• Test the hypothesis.
• Identification of susceptible population.
• Management of the epidemic.
• Formulation of report, communicate findings & recommendation.

48. 1. Verify the diagnosis or causes:
-Careful analysis of the initial reports.
- Confirm diagnosis by performing clinical & lab studies.
-Putting the criteria for case definition.

49. 2. Establish the existence of an epidemic:
Comparing current level (incidence) with past level of the
disease in that locality & population

50. 3. Description of the epidemic as regard TPP:
- Plotting the cases by time of onset (Epidemic curve).
- Plotting the cases by location (Spot map).
- Collect data on the age, sex, etc. of the cases.

51. 4. Develop hypotheses to explain occurrence of the epidemic.
5. Test the hypothesis
6. Identification of susceptible population.
7. Management of the epidemic.
8. Formulation of the report & communicate findings &
recommendation to higher levels in the health system, community
leaders & other local stakeholders.

54. Reservoir Mode of transmission Exposed Host
Case
Carrier
Zoonosis
Man
Animal
Soil
Exit
Exit
Exit
Droplet
Food
Arthropod
 Contact
Vertical
Immunity
&
Susceptibility

58. 1ry“Healthpromotion&
specificprotection”
General
Environmental
sanitation.
Health education.
Health promotion.
Specific
Immunization.
Chemoprophylaxis.
International
preventive measures
Regulations
Quarantine
measures.
Imported goods
Means of
transportation

59. The environment would be free of:
Vehicles of infection: polluted air, water, milk, food, soil
Vector of disease: infection transmitting arthropods
Rodents(including rats): potential reservoir of many infections
Infected animal reservoir (Stray dogs and cats)
Components of sanitary environment:
Proper town,
village or district
planning &
design
Good housing
with suitable
ventilation
Sanitary
collection &
disposal of waste
Food & milk
sanitation
Eradication or
control of insects,
rodents, stray
dogs & cats

60. Health education of the public:
• Health awareness
• Proper KAP related to health, with special consideration of life
style, habits and behavior
Health promotion of the public:
Can be achieved by fulfilling requirements of health:
Physical, mental and social health
Prenatal, natal and post natal care

63. Specific protection of man against causative agents of
infectious diseases by:
Immunization & Chemoprophylaxis

64. International regulations: to prevent transmission of
“Quarantinable diseases” in between countries.
“Cholera, yellow fever & plague”
Quarantine measures: certain animals coming from
endemic or infected areas “Monkey for yellow fever,
Cattle for rift valley fever”
Imported goods: For raw wool, shaving brushes.
Authorized disinfection certificate is needed
Means of transportation: derating certificate for
ships in plague & disinfection of planes coming from
yellow fever endemic areas

66. 2ry“Earlydiagnosis&
prompttreatment”
Control of human
reservoir
Control of cases:
Case finding.
Notification
Isolation
Disinfection
Concurrent
Terminal
Treatment
Release
Control of carriers
Control of contacts
Enlistment
Examination
Surveillance /
Segregation / Isolation
Immunization /
chemoprophylaxis
Control of animal
reservoir
Community control
measures
Drastic measures
Surveillance
Eradication
Elimination

67. Control measures to be taken for existing infectious
disease with the following objectives:
Case finding: detection & diagnosis of cases.
Management of cases & preventing complications.
Measures for contacts & protecting susceptible.
Preventing or minimizing spread of disease.

68. A- Control of cases:
1- Case finding.
2- Notification: Cases of definite or suspected diagnosis
must be notified to local health office.
Value of notification:
To take preventive & control measures
To help tracing sources and channels of infection in
outbreaks
To collect significant statistical data

69. 3- Isolation: at home, hospital or special place
Value of isolation:
To stop activity and movement of case in the community
To protect the case from risk of secondary infection
4- Disinfection: process of destroying pathogenic organisms outside
the body
 Concurrent
Terminal
5- Treatment: kill the infectious agent when it still in the reservoir,
i.e. before it disseminated.
To reduce the communicability of disease, cut short the duration of
illness & prevent the development of 2ry cases.
6- Release: Patient discharge or the formal ending of inpatient care

70. B- Control of carriers:
Carriers may be difficult to control.
It is important to do pre-employment & periodic medical examination
of certain occupational groups e.g food handlers, medical personnel
and personnel serving children

71. C- Control measures for contacts:
 Enlistment: a special list of contacts
 Examination: for case finding
 Surveillance, segregation or isolation:
Surveillance: contacts are put under supervision for IP of disease
meanwhile, they can perform their activities
Segregation: contacts are excluded from school or work but not
isolated e.g measles, enterica & diphtheria
Isolation: contacts of following diseases are isolated:
- Cholera in non-endemic areas - Pneumonic plague & anthrax
 Immunization or chemoprophylaxis

72. Control of animal reservoir:
 Eradication of animal reservoir if applicable.
 Control of farm & pet animals to prevent or minimize
animal-animal or animal man transmission of infection
through sanitary raising, feeding and veterinary care.

73. Community control measures:
Epidemiological study & investigation to trace sources & channels of
infection.
Drastic control measures to be taken if necessary e.g closing
schools
Surveillance
Eradication of infectious disease: is getting rid of causative
organism & consequently of disease: no reported cases & no
reservoir of infection.
Elimination of disease: means that existing endemic infectious
disease is so controlled to reach the level of no reported cases while
causative agent is not necessarily eliminated.

75. Reduce or eliminate long-term impairments & disabilities.
Minimize suffering.
Optimize function.
Prevent further deterioration.
Help complicated cases to cope with their handicap.