The document outlines the chain of infection, which describes the process by which an infectious disease is transmitted. It begins with the infectious agent leaving a reservoir, passing through a portal of exit, being transmitted via a mode of transmission, entering through a portal of entry to infect a susceptible host. The document then defines each link in more detail, including infectious agents, reservoirs, carriers, periods of communicability, and modes of transmission. It provides examples to illustrate key concepts around how infectious diseases are able to spread.

1. CHAIN OF INFECTION
BY/ MAHMOUDSHAQRIA
‫شقريه‬ ‫محمد‬ ‫محمود‬

2. Out lines
-Agent
- Infectively,
- Pathogenicity,
- Virulence
- Antigenicity
- Resistance
- Reservoir of infection
-Carriers
Classification of carriers
- Portal of exit
- Period of communicability

3. - Mode of transmission.
A) direct transmission
B) Indirect transmission:
- Portal of entry
- Incubation period
-Host susceptibility and resistance
Immunity-
-Type of immune defense
-Hazard of immunization
-Herd immunity
Out lines

4. INTRODUCTION
GLOBALLY, MILLIONS OF LIVES (AND BILLIONS OF DOLLARS) ARE LOST
EACH YEAR BECAUSE
OF COMMUNICABLE DISEASES. WHO MILLENNIUM DEVELOPMENT
GOALS CONTINUE
TO FOCUS ON COMMUNICABLE DISEASES AS A MAIN TARGET.
The basic component of every communicable disease transmission is
the chain of infection.
Breaking just one link in the chain of infection means that the
communicable
disease cannot be passed on to another individual.

5. Chain of Infection (Defined)
The “Chain of Infection” describes the process of infection that
begins when an infectious agent leaves its reservoir through a
portal of exit, and is transmitted by a mode of transmission
entering through a portal of entry to infect a susceptible host.

6. Chain of Infection
Agent
Reservoir
Or
carrier
Portal
of EXIT
Mode of
transmi
ssion
Portal of
entry
host

7. 1. Infectious Agent
• Microorganisms that are responsible for
causing disease production:
• Viruses
• Bacteria
• Fungi
• Protozoa and Helminthes
• Parasites
• Prions
Agents are factors that affect disease transmission
through infectivity, pathogenicity, virulence, antigenicity & resistance .

8. Infectively:
the ability of an agent to invade and multiply ( produce infection ) in a susceptible host .
high infectively : measles , polio
Low infectively : leprosy
How to measure the spread of infection ?
Secondary attack rate :
The proportion of exposed susceptible persons who become infected
Secondary attack rate = __________________________ x 100
Number of secondary cases
Number of susceptible

9. Virulence
It refers the ability of organisms to produce sever pathological reaction .
It can measure by : case fatality rate
Pathogenicity :
Is the ability of the organisms to produce specific clinical reaction after infection
It refer to the proportion of infected persons who develop clinical disease
It can measured by :
Ratio of clinical to subclinical cases =
Clinical cases
sub clinical cases
Case of fatality rate =
Total number of deaths from a disease
Total number of cases of disease
x 100

10. Antigenicity ( immunogenicity ) :
the ability of an organism to produce specific immunity ( antibodies , antitoxin )
it can be measure by : reinfection rate Second attacks are rare in measles , mumps and
chicken pox
Reinfection occurs as in case of common cold , syphilis and gonorrhea
Resistance: the ability of an agent to survive adverse environmental condition
Examples :
Low environmental resistance as gonococci and influenza VIRUSES ARE typically
fragile
Moderate environmental resistance as salmonella
High environmental resistance as mycobacterium tuberculosis

11. 2. Reservoirs of Infection
The reservoir of an infectious agent
is the habitat in whichthe infectious agent
normally lives and multiplies.
Types of reservoirs:
• Humans
• Animals
• Environment

12. Human reservoir :
Two types of human reservoir exist :
1. cases: persons with symptomatic illness
- typical : A case suffer from an infectious disease , discharges microorganisms , and is
a source of microorganisms
- Atypical : A case which can`t easily diagnosed or isolated.
Subclinical cases : the person is infected but the disease is not clinically manifested.
2. Reservoirs of Infection ( cont )

13. 3. Carriers
They are infected persons, apparently healthy with no symptoms, having the organisms in
their bodies, excrete them with their discharges and disseminate infection to others
Carriers are dangerous because:
They don't show any clinical manifestations so they carry normal life.-
-Carriers are not diagnosed and they are not know by others, it is difficult to discover them.
-The carrier and his contacts aren't aware of their condition so they take no percussion.
Large number of carriers and exceed very much the
number of cases-
-Carrier in some disease are dangerous because of nature of their work e.g. food handler and
school personells
-Carriers of some disease may remain infective for long period or sometimes for lifelong e.g.
incubatory carrier of HIV and chronic carrier as hepatitis B and typhoid

14. Classification of carriers
According to relation to case , carrier are to classified to :
1.Incubatory carrier : the case become infective before the onset of the disease
During the incubation period e.g in the last few
days ( cholera and typhoid )
In the last few weeks ( one weak before onset in viral A hepatitis )
2. convalescent carriers : the recoverd cases continue to excrete the infective agents during
the period of convalescence e.g typhoid , diphtheria and cholera
3. contact carriers :contact of cases having moderate immunity may be infected and get rid of
infection within maximally two weeks eg typhoid
4. Healthy carriers : those infected persons from polluted environment such as contaminated
food and water this occurs in endemic infectious disease and they are usually temporary
carriers

15. B) According to period of carrier's state:
1. Transient carriers : they are infective only for days e.g. last few days of incubation period e.g. cholera
2. Temporary carriers : they are infective for few weeks up to few months e.g. viral A hepatitis last week of I.P
and viral B hepatitis 3 weeks : 3 months
3. Chronic carriers : who are still infective for years e.g. chronic carriers in typhoid and hepatitis B
C) According to the foci of infection:
1. Respiratory system : who carry the microorganisms in the respiratory tract
Throat e.g. diphtheria , streptococcus haemolyticus & staphylococcus aureus
nose : diphtheria staphylococci
nasopharynx : meningococci , pneumococcal
2. Gastro-intestinal tract :
Gall bladder : e.g. salmonella typhi and para typhi
Small intestine : salmonella typhi and para typhi
3. urinary tract : s.typhi and paratyphi
4. skin lesion or under the nails : as staphylococcus aureus in food poisoning

16. D) according to discharge that carry organisms outside the body
1. Respiratory discharge: from the nose and throat through coghing sneezing spitting etc ,
staph , stept , influenza ,meningitis .
2. faecal carriers in intestinal infections e.g. typhoid , paratyphoid , amaebiasis
3. urinary carrier as in typhoid and paratyphoid
4. skin discharge , organisms may be present in skin leasion as in skin disease .
E) according to flow of the organisms outside the body .
Continuous or intermittent
N.B : the following disease have no carriers :
- influenza -whooping cough -TB - measles -herpes zoster

17. B) animal reservoirs :
Zoonoses : infectious disease that transmitted under normal conditions from vertebrate
animals to humans ( with humans as incidental host )
Role of animal in zoonoses :
1. Animal are only reservoirs of infection ( strictly zoonotic diseases ) no man to man infection
as in plague , brucellosis and q fever
2. both animals and man are reservoirs of infection and may be animal to man or man to man
as:
- yellow fever : monkeys and man are the reservoirs
- salmonellosis and salmonella food poisoning : man and some animal species
( cattle , swine , poultry , rats ) are the reservoirs of non typhoidal salmonellae
3. animal may be intermediated host of parasitic disease and man is the definitive host e.g.
hydatid disease
4. animal may be the definitive host man is affected by intermediate stage of the parasite as
taenia saginata

18. C) Environmental reservoirs
Plants soil and water
Soil : agent live and multiply in the soil example : tetanus and anthrax spores and
fungal agent causing histoplasmosis.
Pools of water : the primary reservoir of legionnaires bacillus.

19. 4 – portal of exit
Is the path by which an agent leave the source host.
The organisms leave the body of the reservoir through :
1. Respiratory orifices: during forced expiratory acts such as sneezing ,
coughing as influenza , streptococci and measles.
2. Gastro intestinal tract: the organisms may found in vomitus e.g. cholera or fecal discharge as gonorrhea and
syphilis
3. Genitourinary tract: urine as enterica and urinary tract infection and genital discharge as gonorrhea and syphilis.
4. Discharge of skin and mucous membrane : as in infected wounds , skin eruption , small pox and chicken pox eye
discharge as in infective conjunctivitis
5. inutero transmission , from pregnant mother to her fetus e.g. German measles , cytomegalovirus and
toxoplasma
6. blood: organisms & parasites in blood can find exit through contaminated syringes and during blood transfusion
e.g. HIV , viral hepatitis and also exit through bites of insects
7. others : Milk as HIV , HCV cytomegalovirus and tears as HIV

20. 5- period of communicability
Infectively may be terminated by clinical cure as in small pox , chicken pox , and measles
In disease having carrier state , infectivity may star the last few days or weeks of incubation
period ( temporary carriers ) as viral hepatitis and may extend during during the convalescent
stages as in viral B hepatitis and typhoid ( chronic carrier )

21. 6. Modes of Transmission Direct
A. direct transmission : there is essentially immediate transfer at the
agent from a reservoir to a susceptible host
There are five methods of direct transmission
1. direct transmission due to the agent being within a reasonably close distance of host as
occurs in ( direct droplet infect on )
2.contact of host skin or mucous membrane with infectious agent in a living tissue e.g.
sexually transmitted disease
3.contact of skin or mucous membrane with the infectious form of the agent
contained in inanimate environment e.g. hookworm (infective form in soil )
4.inoculation of the agent , directly from the reservoir into the skin or
mucous membrane as rabies
5. vertical transmission from mother to child or through the placenta e.g. HIV , syphilis , torch
, toxoplasma , rubella , cytomegalovirus etc.

22. B) Indirect transmission:
An agent is carried from a reservoir to a susceptible host by an intermediary
agency :
There are four methods of indirect transmission
Airborne transmission : suspended air particles
Vehicle borne : inanimate vehicle
Finger ( hand to mouth )
Vector borne : animate vector

23. A).Airborne transmission : occurs by particles that suspended in air (two
types)
1.Dust : result from re suspension of particles that have settled on floor or
bedding as well as infectious particles blown from the soil by the wind .
example fungal spores , legionnaires disease , Q fever , TB , nosocomial
infection.
2.droplet nuclei : they represent the dried residue of droplets that have been
coughed or sneezed into the air
They are very tiny particles less than 5 microns in size and may remain
suspended in the air for long periods.
Examples:
TB is transmitted more often indirectly through droplet nuclie than directly
through direct droplet spread.

24. B). VEHICLE –BORNE : An infectious agent is carried from a reservoir to a susceptible host by
an inanimate intermediary . vehicles included :
1) contaminated food and drink.
2) injectable biologic products (blood , blood products , drugs , vaccine , diluents ) e.g. HIV ,
viral hepatitis , syphilis and malaria
3) fomites ( inanimate objects of general use such as utensils , toys, tooth brushes ,
handkerchiefs , bedding , or surgical instruments ) .
3. fingers ( hand to mouth ) : finger from a very important mode of indirect transmission
4. vector borne indirect transmission.
.

25. - A vector is aliving invertebrate which transfers the infection agent from the
source of infection to another susceptible host.
- mechanical transmission : the agent does not multiply or undergo
physiologic changes in the vector
- Biologic transmission :
- propagative : when the agent undergoes multiplication within the vector
before it is transmitted
- developmental : when the agent undergoes changes within the vector
before it transmitted .
- cyclo-developmental : when the agent undergoes changes and
multiplication within the vector before it is transmitted

26. Arthropod borne infection
Some insects of medical importance :
Mosquitoes : which include different species.
Anopheles : ( pharaensis , sergenti , multicolor and Gambia ) transmit malaria
Culex pipiens : transmit filariasis , rift valley fever , encephalitis
Aedes Aegypti : transmit yellow fever and dengue fever.

27. 7- portal of entry
An agent enters a susceptible host through a portal
of entry .
The portal of entry must provide access to tissue in
which the agent can multiply or a toxin can act.
Often , organisms use the same portal to enter a
new host that they use to exit the source host.

28. 8 incubation period
The period from the entry of infectious agent (or its toxin ) into the human body until the
earliest clinical manifestations of the disease are apparent . it depends on :
-the portal of entry
-the rate of growth of the organism in the host
-the dose of the infectious agent
-the host resistance
2:6 hrsStaph .food poisoningThe shortest I.P.
6 :12 hrsSalmonella food poison
12 :36 hrsBotulism
10d up to 1 yearRabiesThe longest I.P.
3 :5 yearsAIDS
Up to 10 years.Leprosy

29. 9. Susceptible Host
The host is a person or other living
organism that can be infected by an
infectious agent under normal
conditions.
Susceptibility of a host depends on:
• General factors ( age , sex , education …etc. )
• Genetic factors
• Specific acquired immunity
A susceptible host is the final link in the chain of
infection.

30. Treating high risk
patients
(immunosuppressed,
old, young, surgery,
burns, diabetics, CVD)
Treating underlying diseases
Employee health
nvironmental sanitation
Disinfection
Reservoir management
(eg. culling)
Rapid accurate organism identification
Use of PPE
Hand washing
Control of secretions-skin droplets
Waste sample disposal
Personal etiquette (sneezing)
Wound-Catheter care
Sanitation
Aseptic techniques
Hand washing
Use of PPE
Hand washing
Proper food handling
Sterilization-disinfection
Social distancing
Public Health Actions to Break
the Chain
of Infection

31. Public Health Actions to Break the Chain of Infection at the
Portal of Exit
Breaking the LinkPortal of Exit
Wear a mask
Do not talk directly into patient’s face
Stay home if sick
Practice cough etiquette
Use hand hygiene
Respiratory Tract. Agents leave by
droplets or sprays (coughing,
sneezing, talking, singing or
breathing.
Handle body secretions properly
Use PPE
Perform good housekeeping
Use hand hygiene
Gastrointestinal Tract. Agents leave
by body secretions (e.g. stool and
vomit).
Dispose of wound dressings properly
Use personal protective equipment
Use hand hygiene
Skin. Agents leave by wound drainage
or through skin lesions.
Safe handling of sharps
Use PPE during risk of exposure to blood
Diligence when processing specimens
Use hand hygiene
Blood. Transmission may occur by
transfusion or use of contaminated
sharps.

32. Public Health Actions to Break the Chain of Infection at the
Portal of Entry
Breaking the LinkPortal of Entry
Wear a mask/respirator
• Maintain good ventilation
• Isolate those with respiratory symptoms
• Good respiratory hygiene/etiquette practices
• Use hand hygiene
Respiratory Tract.
Agents can be inhaled (droplet nuclei)
Dispose of body excretions carefully
• Vigilant food handling
• Perform good housekeeping
• Wear appropriate personal protective equipment
• Use hand hygiene
Gastrointestinal Tract.
Agents can enter a new host when
food or water contaminated by feces
is ingested (fecal/oral route).
Use face shield during high risk procedures
• Carry out good housekeeping
• Use hand hygiene
Mucous membranes.
Agents absorbed through exposed
eyes, nose and mouth.
Dispose of wound dressings carefully and properly
• Wear personal protective equipment
• Maintain healthy intact skin
• Use hand hygiene
Skin.
Agents enter by wound drainage or
skin secretion contact.

33. Public Health Actions to Break the Chain of Infection
Will depend on the infectious agent. General actions include:
• Hand washing and Hand Hygiene Programs
• Covering nose when coughing or sneezing
• Social distancing strategies
• Infectious disease hazard communication
• Immunization – yearly flu shot
• Personal Protective Equipment (eg. gloves, gowns, visors, N95
masks). Use, fit, maintenance and replacement
• Hand washing and Hand Hygeine Programs
• Environmental controls (eg. isolation, negative pressure rooms)
• Surface cleaning and disinfection (eg. sanitizing, UV light)
• Ongoing updates, education and training for infectious disease
• Targeting vectors (eg. larvacides for WNV,
trail brush cutting)
• Reservoir management (culling programs)
Individual
Institutional-
Organizational
Municipal-
Provincial

34. Immunity
Immunity is the defense mechanism tat protect the body from micro organisms and
potentially harmful agent
Immune response
Secondary immune responsePrimary immune response
Re exposure to same Ag
Characters
-short latent period due to early Abs
production
-large amount of Abs
-Abs last for long duration
Abs have high binding capacity with Ag
Importance
Used in immunization programmer every few
weeks to prolonged duration of immunization
Exposure to the Ag for the 1st time
Characters
-Long latent period : lasts for 10 days due to
delayed AB production
-small amount of Abs
-Abs last for short duration ( disappear
rapidly)
-Abs have low binding capacity with Ag
Importance
Stimulate reticuloendothelial system to
produce memory cells

35. Type of immune defense :
1)general non specific (innate) defense mechanism of the body.
1-mechanical barrier and surface secretion of the body
Healthy and intact skin and mucous membrane prevent invasion of most of micro
organism
Sticky mucous that cover the mucous membranes trap any foreign material
And microorganism
Eye : blink reflex and tear can prevent infection .
GIT : saliva, mucous secretion (lysozyme) and gastric acidity and diarrhea prevent
infection
Respiratory system :cilia, coughing , and sneezing reflex sweep out any foreign material
and micro organism
Genitourinary secretion : acidic ph of vagina inhabits growth of micro organisms

36. 2.normal bacterial flora
Produce bacteriocins and acids that destroy micro organisms
They complete with pathogens for essential nutrients
3. humoral defense mechanism such as
Lysozyme lead to lysis of Bactria
Interferon which is non specific defense mechanism against viral infection
Complement which is considered a natural innate immunity
Plasma can also dilute toxins
4.Cellular defense mechanism : as phagocytosis which engulf particles entering the blood
5.inflammatory reaction : vasodilatation , increase vascular permeability and cellular
infiltration can resist invading agents.

37. Acquire immunity
Artificially acquireNaturally acquire
activepassiveactivepassive
By administrateby
Vaccine
Live or killed
Human
immunoglobulins
Disease attackMother through
placenta
toxoidAnimal anti sera
Anti toxins
Subclinical infectionMother through milk
A)naturally acquire immunity
1- passive
A) transplacenal
B) mothers milk :

38. 2-Active
A)subclinical infection :
Exposure to subclinical infection gives immunity against endemic infection disease of
community as : poliomyelitis and diphtheria
B) clinical infection :
Some disease have no sub clinical manifestation but represent clinically only as : measles ,
chicken pox , syphilis and gonorrhea . attack of infection disease are followed by variable
degrees of immunity which may be:
absolute (no second attack) after yellow fever only
Solid (high degree , long lasting) as after measles , German measles , chicken pox and small
pox
Moderate : as after enteric fever
weak ( short lasting ) as after influenza as multiplicity of strains of the virus , continuous
mutation and change in antigenicity so there is repeated attack of influenza

39. B) artificial acquired immunity :
1- passive immunization (seroprophylaxis )
It is the inoculation of the immune serum that contain already manufactured ,
immunoglobulins or lymphocytes to induce or cellular immunity .
Types : animals or human preparation
typedisadvantagesAdvantage
Anti toxic sera e.g.
diphtheria & tetanus
Anti snake sera
Antiviral sera as
antirabies serum
Given in large doses
Give short protection
1:2 week
May lead to server
hypersensitivity
reaction
Relatively cheap
Usually available

40. Human immunogloblobulin ( homologous ) : prepared from human sources
Disadvantage : relatively expensive and not constantly available.
Advantage : used in small dose . gives immediate immunity for longer period 30 : 50 days
safe as it dose not lead to hypersensitivity reactions
Type:
Specific human immunoglobulinNormal human immunoglobulin (NHI)
Prepared from the donner who vaccinated
against communicable disease or carrier of
specific infections
Uses : in prevention of viral disease
HBIG .06ml/kg
Prevention of varicella zoster infection
Prevention of rabies 20iu /kg
Prevention of tetanus 250 unite for
prophylaxis and 3000: 6000 unit for therapy
Prepared from plasma of hepatitis B and C
and HIV free donors in endemic area
Uses : effective in prophylaxis of measles ,
rubella , poliomyelitis & viral A hepatitis
Sero prevention
In early exposure & proper dose
Sero attenuation
In late exposure or small dose

41. Artificial acquire immunity ( active vaccination )
This type of immunity is induced after vaccination by different type of vaccines
1-Live vaccine : small pox vaccine prepared from cow pox virus
2-Live attenuated vaccine : vaccine prepared from attenuated organisms
They should not be given to person with immunodeficiency
It is more potent than killed vaccine
It usually given as one dose
Examples : BCG , measles , rubella , mumps , sabin oral polio vaccine and yellow fever vaccine
3-Killed or inactive vaccine : organisms is killed by heat or chemicals & injected to
stimulate active immunization
Generally , killed vaccine are less effective than live vaccine

42. 4) cellular fraction vaccine : prepared from extra cellular fraction vaccine : prepared from
extra cellular fraction e.g. meningococcal vaccine
Artificial acquire immunity ( active vaccination ) cont
5) surface antigen vaccine for viral B hepatitis of two type :
Derived from the ( HBsAg ) from plasma of chronic carriers.
6)Poly valent vaccine : which contain the difference strain of the same organisms as in polio
virus type 1 ,2 ,3
7)Combination of vaccine ( mixed or combined ) : more than one immunizing agent in the
vaccine are combined to simplify administration of different immunization agent at one
time and to reduce cost e.g.
DPT : diphtheria , pertussis , tetanus
DPT Salk : quadriple vaccine diphtheria , pertussis , tetanus and salk polio vaccine
DT : Diphtheria , tetanus.
MMR : measles , mumps , rubella

43. Hazard of immunization
-General reaction : fever , malaise , headache and other constitutional
manifestation.
-General infection : contaminated needles and syringes as HIV , HBV , HCV CMV
& pyogenic infection
-Local reaction : e.g. pain swelling , redness tenderness and abscess at the site of
infection
-Hypersensitivity reaction : in case of anti sera or viral vaccine prepared from
embrynated egg , e.g. influenza and mumps , in the form of serum sickness or
anaphylactic shock
-Neurological involvement : as post vicinal encephalitis or neuroparalytic
accidents after rabies vaccine , encephalopathy as in pertussis vaccine

44. Contraindications to child immunization .
There are almost no contra indication to vaccination
It is safe to immunize children even if they are moderately ill
If you delay vaccination due to mild illness many children will contract the target disease
Children with mild illness should be immunized as usual
Children with malnutrition can develop good immunity so they are immunized as usual
Absolute contraindication are :
Very sever ill children who who need to hospitalized or children who have very high fever :
vaccination should be delayed
If a child has had sever reaction from D.P.T ( convulsion or shock ) do not give the child any
more doses of D.P.T and give him D .T
B.C.G is only vaccine which shouldn't be given to children with clinically apparent AIDS or
immune deficiency disease .

45. Herd immunity
It is state of immunity of a group or community
Also it is the resistance of the a group to invasion and spread of an infectious agent
Based on immunity of a high proportion of individual members of the group
Characteristics of herd immunity
If a large percent of population is immune the entire population is likely to be protected
Once a high proportion of all people in the community are immune the likelihood is small that
an infected person will encounter a susceptible person
Herd immunity operate optimally when there is random mixing of the population ( no
clustering of susceptible persons)