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Dr. Dalia El-Shafei
Assistant professor, Community Medicine Department,
1) To recognize the infectious cycle (epidemiological triad)
2) To understand the chain of infection.
3) To understand the natural history of the disease
• Disease is the result of
forces within a dynamic
system called an
which consisting of:
• To put preventive & control measures, limit their
spread in the community and limit their complications
Communicable diseases: are diseases transmitted from
one case to another. There is a cycle for transmission of
infection from one host to another.
• Production, Metabolism, Motility, Production of toxin.
agent related to
• Infectivity: Ability of an organism to invade & infect a
• Pathogenicity: Ability to cause disease.
• Virulence: Ability to cause serious complications /death.
• Antigenicity: Ability to stimulate host production of
Source of infection
or object from
which agent passes
Natural habitat, in
which an agent
lives, grows &
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)”.
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
Carriers “the most dangerous source of infection”
Subclinical or in apparent infection Atypical cases
Portal of exit
• 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
• Soil can also act as reservoir of infection (e.g. soil may
harbor agents that cause tetanus & anthrax).
• Food: meat, milk, vegetables.
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
• Directly: droplets
cough, spitting” (short
• Indirectly: droplet
nuclei or contaminated
Droplet or air
• Directly: water,
• Indirectly: flies,
• Intrauterine infection
as German measles,
• Direct contact with case or infected
• Indirect: clothes, fomites, dust, water
pool. Infection by contaminated
syringes occurs through skin
Contact method “Skin
& mucous membrane”
• Biological transmission: development
of new stages of organism occur inside
• Mechanical: vector just carry
organism from case to new host
• Route the agent uses to get into the new host.
• In general, portal of entry is similar to portal of exit
Intrinsic factors that influence an individual's exposure,
susceptibility, or response to a causative agent
Age Sex Pregnancy Nutrition
Factors affecting host immunity
Herd immunity (immunity of the community)
Describes the immunity level that is present in a population
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.
Types of immunity
• Mechanisms that prevent entrance of agents to the body
• Intact skin
• Cough, sneezing reflexes
• Gastric juices
• Normal bacterial flora
• Secretions: saliva, tears.
• 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”
Active Passive (maternal immunity)
After infection, the
body forms antibodies
against that infection.
- Solid immunity:
- Short time
Antibodies from mothers during pregnancy
are transmitted to infant.
It depends on diseases the mother had
exposed to before pregnancy to form
No cellular immunity is transmitted because
of placental barriers.
Antibodies also are present in colostrum
breast milk secreted on the 1st days after
Naturally Acquired Immunity
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
• Give lifelong immunity.
• Contains killed agents
but still retain their
antigenicity as typhoid,
• Give short time or low
level of immunity.
Whole organism “polio & measles”
Capsule of organism “meningitis”
Surface antigen of virus “HBV”
Toxin secreted by organism
“diphtheria & tetanus”
Intradermal “BCG” Oral “Polio”
Route of Vaccination
• 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
Causes of failure of Active Immunization
Ready-made “Ig” given for rapid protection after
exposure to infection for prevention or attenuation of
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
Administration of antimicrobial drug before exposure or just
after exposure to prevent occurrence of disease (not for
May be the basic preventive measure: Penicillin for rheumatic
fever, tetracyclines for cholera, INH for TB, rifampicin for
• Cost: expensive compared to expected potential benefit”
• Side effects of drugs
• Development of drug resistant strains
• Temporary protection.
Artificial acquired passive immunity
• 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.
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
Internal defense mechanism
• Cellular & humeral immunity.
• If external defense mechanism
can’t eliminate agent; internal
defense mechanism continue
• Isolation by fibrocytes
• Macrophage phagocytosis
• Antibody reaction
Importance of IP:
• Tracing the source of infection & contact
• Period of surveillance
• Identification of point source or propagated epidemics
• Prognosis e.g. in rabies & tetanus, the shorter the IP is, the
worse the prognosis of disease.
Differs from disease to another & from one case to another.
• Infected cases have no common source of infection.
• Spread all over far away areas not related to each other.
• Spread of infection in confined place as school or camp.
• Usually there is common source for infection, if
controlled the incidence of infection decreases.
• 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
• Constant presence of disease within a
given geographic area or population
• Spread of infection all over the world or
in many countries e.g. avian flu, Aids.
• Spread of Zoonotic disease in large scale
among animals e.g. cow mad disease.
Common source epidemic
• All cases diagnosed are
infected from a common
source at certain point of
• The resultant cases all
develop with one IP of
disease e.g. epidemic of
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.
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.
• 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.
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.
2. Establish the existence of an epidemic:
Comparing current level (incidence) with past level of the
disease in that locality & population
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.
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.
Reservoir Mode of transmission Exposed Host
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:
village or district
disposal of waste
Food & milk
control of insects,
dogs & cats
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
Specific protection of man against causative agents of
infectious diseases by:
Immunization & Chemoprophylaxis
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
Control of human
Control of cases:
Control of carriers
Control of contacts
Segregation / Isolation
Control of animal
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.
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
To collect significant statistical data
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
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
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
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
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.
Community control measures:
Epidemiological study & investigation to trace sources & channels of
Drastic control measures to be taken if necessary e.g closing
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.
Reduce or eliminate long-term impairments & disabilities.
Prevent further deterioration.
Help complicated cases to cope with their handicap.