The document defines key concepts in epidemiology and communicable disease transmission. It discusses: - Epidemiology as the study of health-related events in populations and applying findings to control health problems. - The epidemiologic triad of host, agent, and environment factors influencing disease. - Communicable diseases as illnesses transmitted directly or indirectly between humans, animals, or the environment. - The importance of studying communicable disease epidemiology to understand changing disease patterns and potential infectious causes of chronic diseases.

1. Principles ofPrinciples of
Communicable DiseasesCommunicable Diseases
EpidemiologyEpidemiology

2. ObjectivesObjectives
 Definition of epidemiology
 The epidemiologic triad
 Definition of communicable diseases
 Importance of studying
communicable diseases epidemiology
 Terminology
 Dynamics of disease transmission
(chain of infection):
– Human reservoir or source
– Modes of transmission
– Susceptible host

3. Definition of EpidemiologyDefinition of Epidemiology
Epidemiology is the study of the
distribution and determinants of
health-related states and events
in populations, and the
application of this study to
control health problems (Last,
1983(.

4. Epidemiologic triadEpidemiologic triad
Host
Agent Environment
•Biological agents
•Physical agents
•Chemical agents
•Nutrient agents
•Mechanical agents
•Social agents
•Physical environment
•Biological environment
•Social environment
•Demographic characteristics
•Biological characteristics
•Socioeconomic characteristics

5. Infectious Disease ModelInfectious Disease Model
Host
Pathogen
Environment
disease

6. Definition of communicableDefinition of communicable
diseasesdiseases
 A communicable disease is an illnessA communicable disease is an illness
due to a specific infectiousdue to a specific infectious
(biological) agent or its toxic(biological) agent or its toxic
products capable of being directly orproducts capable of being directly or
indirectly transmitted from man toindirectly transmitted from man to
man, from animal to man, fromman, from animal to man, from
animal to animal, or from theanimal to animal, or from the
environment (through air, water,environment (through air, water,
food, etc..) to man.food, etc..) to man.

7. Importance of Studying CommunicableImportance of Studying Communicable
Diseases EpidemiologyDiseases Epidemiology
 Changes of the pattern of infectiousChanges of the pattern of infectious
diseasesdiseases
 Discovery of new infectionsDiscovery of new infections
 The possibility that some chronicThe possibility that some chronic
diseases have an infective origin.diseases have an infective origin.

8. Terminology and DefinitionsTerminology and Definitions
 InfectionInfection
 ContaminationContamination
 InfestationInfestation
 Contagious diseaseContagious disease
 Incidence andIncidence and
prevalence ofprevalence of
infectious diseasesinfectious diseases
 EpidemicEpidemic
 EndemicEndemic
 HyperendemicHyperendemic
 holoendemicholoendemic
 PandemicPandemic
 ExoticExotic
 SporadicSporadic
 Attack rateAttack rate
 Primary/secondaryPrimary/secondary
casescases
 Zoonosis, epizooticZoonosis, epizootic
and enzooticand enzootic
 Nosocomial infectionNosocomial infection
 Opportunistic infectionOpportunistic infection
 EradicationEradication
 EliminationElimination

9. Terminology and Definitions (contTerminology and Definitions (cont.(.(
 VirulenceVirulence
 Reproductive rate ofReproductive rate of
infectioninfection
 HostHost
 Vector (source)Vector (source)
 ReservoirReservoir
 Incubation periodIncubation period
 Infectivity periodInfectivity period
 Serial intervalSerial interval
 Latent periodLatent period
 TransmissionTransmission
Probability ratioProbability ratio

10. InfectionInfection
 Infection is the entry and development orInfection is the entry and development or
multiplication of an infectious agent in themultiplication of an infectious agent in the
body of man or animals. An infection doesbody of man or animals. An infection does
not always cause illness.not always cause illness.
 There are several levels of infectionThere are several levels of infection
(Gradients of infection):(Gradients of infection):
– ColonizationColonization (S. aureus in skin and normal(S. aureus in skin and normal
nasopharynx)nasopharynx)
– Subclinical or inapparent infectionSubclinical or inapparent infection (polio)(polio)
– Latent infectionLatent infection (virus of herpes simplex)(virus of herpes simplex)
– Manifest or clinical infectionManifest or clinical infection

11. contaminationcontamination
 The presence of an infectious agentThe presence of an infectious agent
on a body surface, on or in clothes,on a body surface, on or in clothes,
beddings, toys, surgical instrumentsbeddings, toys, surgical instruments
or dressings, or other articles oror dressings, or other articles or
substances including water and foodsubstances including water and food

12. InfestationInfestation
 It is the lodgment, development andIt is the lodgment, development and
reproduction of arthropods on thereproduction of arthropods on the
surface of the body or in thesurface of the body or in the
clothing, e.g. lice, itch mite. Thisclothing, e.g. lice, itch mite. This
term could be also used to describeterm could be also used to describe
the invasion of the gut by parasiticthe invasion of the gut by parasitic
worms, e.g. ascariasis.worms, e.g. ascariasis.

13. Contagious diseaseContagious disease
 A contagious disease is the one thatA contagious disease is the one that
is transmitted through contact.is transmitted through contact.
Examples include scabies, trachoma,Examples include scabies, trachoma,
STD and leprosy.STD and leprosy.

14. HostHost
 A person or an animal that affordsA person or an animal that affords
subsistence or lodgement to ansubsistence or lodgement to an
infectious agent under naturalinfectious agent under natural
conditions. Types include: anconditions. Types include: an
obligate host, definitive (primary)obligate host, definitive (primary)
host, intermediate host and ahost, intermediate host and a
transport host.transport host.

15. Vector of infectionVector of infection
 An insect or any living carrier thatAn insect or any living carrier that
transports an infectious agent fromtransports an infectious agent from
an infected individual or its wastes toan infected individual or its wastes to
a susceptible individual or its food ora susceptible individual or its food or
immediate surroundings. Bothimmediate surroundings. Both
biological and mechanicalbiological and mechanical
transmissions are encountered.transmissions are encountered.

16. ReservoirReservoir
 Any person, animal, arthropod,Any person, animal, arthropod,
plant, soil, or substance, or aplant, soil, or substance, or a
combination of these, in which ancombination of these, in which an
infectious agent normally lives andinfectious agent normally lives and
multiplies, on which it dependsmultiplies, on which it depends
primarily for survival, and where itprimarily for survival, and where it
reproduces itself in such a mannerreproduces itself in such a manner
that it can be transmitted to athat it can be transmitted to a
susceptible host. It is the naturalsusceptible host. It is the natural
habitat of the infectious agent.habitat of the infectious agent.

17. Incidence and prevalence ofIncidence and prevalence of
infectious diseasesinfectious diseases
 Incidence of an infectious disease: number ofIncidence of an infectious disease: number of
new cases in a given time period expressed asnew cases in a given time period expressed as
percent infected per year (cumulative incidence)percent infected per year (cumulative incidence)
or number per person time of observationor number per person time of observation
(incidence density).(incidence density).
 Prevalence of an infectious disease: number ofPrevalence of an infectious disease: number of
cases at a given time expressed as a percent at acases at a given time expressed as a percent at a
given time. Prevalence is a product of incidence xgiven time. Prevalence is a product of incidence x
duration of disease, and is of little interest if anduration of disease, and is of little interest if an
infectious disease is of short duration (i.e.infectious disease is of short duration (i.e.
measles), but may be of interest if an infectiousmeasles), but may be of interest if an infectious
disease is of long duration (i.e. chronic hepatitisdisease is of long duration (i.e. chronic hepatitis
B).B).

18. EpidemicEpidemic
 ““The unusual occurrence in aThe unusual occurrence in a
community of disease, specific healthcommunity of disease, specific health
related behavior, or other healthrelated behavior, or other health
related events clearlyrelated events clearly in excess ofin excess of
expected occurrenceexpected occurrence””
 (epi= upon; demos= people)(epi= upon; demos= people)
 Epidemics can occur upon endemicEpidemics can occur upon endemic
states too.states too.

19. EndemicEndemic
 It refers to the constant presence ofIt refers to the constant presence of
a disease or infectious agent within aa disease or infectious agent within a
given geographic area or populationgiven geographic area or population
group. It isgroup. It is the usual or expectedthe usual or expected
frequency of diseasefrequency of disease within awithin a
population.population.
 (En = in; demos = people)(En = in; demos = people)

20. Hyperendemic and holoendemicHyperendemic and holoendemic
 The termThe term ““hyperendemichyperendemic”” expresses thatexpresses that
the disease is constantly present at highthe disease is constantly present at high
incidence and/or prevalence rate andincidence and/or prevalence rate and
affects all age groups equally.affects all age groups equally.
 The termThe term ““holoendemicholoendemic”” expresses a highexpresses a high
level of infection beginning early in life andlevel of infection beginning early in life and
affecting most of the child population,affecting most of the child population,
leading to a state of equilibrium such thatleading to a state of equilibrium such that
the adult population shows evidence of thethe adult population shows evidence of the
disease much less commonly than do thedisease much less commonly than do the
children (e.g. malaria)children (e.g. malaria)

21. Pandemic and ExoticPandemic and Exotic
 An epidemic usually affecting a largeAn epidemic usually affecting a large
proportion of the population, occuring overproportion of the population, occuring over
a wide geographic area such as a sectiona wide geographic area such as a section
of a nation, the entire nation, a continentof a nation, the entire nation, a continent
or the world, e.g. Influenza pandemics.or the world, e.g. Influenza pandemics.
 Exotic diseases are those which areExotic diseases are those which are
imported into a country in which they doimported into a country in which they do
not otherwise occur, as for example,not otherwise occur, as for example,
rabies in the UK.rabies in the UK.

22. SporadicSporadic
 The word sporadic meansThe word sporadic means ““scatteredscattered
aboutabout””. The cases occur irregularly,. The cases occur irregularly,
haphazardly from time to time, andhaphazardly from time to time, and
generally infrequently. The cases are fewgenerally infrequently. The cases are few
and separated widely in time and placeand separated widely in time and place
that they show no or little connection withthat they show no or little connection with
each other, nor a recognizable commoneach other, nor a recognizable common
source of infection e.g. polio,source of infection e.g. polio,
meningococcal meningitis, tetanusmeningococcal meningitis, tetanus……..
 However, a sporadic disease could be theHowever, a sporadic disease could be the
starting point of an epidemic when thestarting point of an epidemic when the
conditions are favorable for its spread.conditions are favorable for its spread.

23. Attack rates and primary/secondaryAttack rates and primary/secondary
casescases
 Attack rate: proportion of non-immuneAttack rate: proportion of non-immune
exposed individuals who become clinicallyexposed individuals who become clinically
ill.ill.
 Primary (index)/secondary cases: ThePrimary (index)/secondary cases: The
person who comes into and infects aperson who comes into and infects a
population is the primary case. Those whopopulation is the primary case. Those who
subsequently contract the infection aresubsequently contract the infection are
secondary cases. Further spread issecondary cases. Further spread is
described as "waves" or "generations".described as "waves" or "generations".

24. Zoonosis, epizootic and enzooticZoonosis, epizootic and enzootic
 Zoonosis is an infection that isZoonosis is an infection that is
transmissible under natural conditionstransmissible under natural conditions
from vertebrate animals to man, e.g.from vertebrate animals to man, e.g.
rabies, plague, bovine tuberculosisrabies, plague, bovine tuberculosis……....
 An epizotic is an outbreak (epidemic) ofAn epizotic is an outbreak (epidemic) of
disease in an animal population, e.g. riftdisease in an animal population, e.g. rift
valley fever.valley fever.
 An Enzotic is an endemic occurring inAn Enzotic is an endemic occurring in
animals, e.g. bovine TB.animals, e.g. bovine TB.

25. Nosocomial infectionsNosocomial infections
 Nosocomial (hospital acquired)Nosocomial (hospital acquired)
infection is an infection originating ininfection is an infection originating in
a patient while in a hospital ora patient while in a hospital or
another health care facility. It has toanother health care facility. It has to
be a new disorder unrelated to thebe a new disorder unrelated to the
patientpatient’’s primary condition.s primary condition.
Examples include infection of surgicalExamples include infection of surgical
wounds, hepatitis B and urinary tractwounds, hepatitis B and urinary tract
infetions.infetions.

26. Opportunistic infectionOpportunistic infection
 This is infection by organisms thatThis is infection by organisms that
take the opportunity provided by atake the opportunity provided by a
defect in host defense (e.g.defect in host defense (e.g.
immunity) to infect the host and thusimmunity) to infect the host and thus
cause disease. For example,cause disease. For example,
opportunistic infections are veryopportunistic infections are very
common in AIDS. Organisms includecommon in AIDS. Organisms include
Herpes simplex, cytomegalovirus,Herpes simplex, cytomegalovirus,
 M. tuberculosisM. tuberculosis……..

27. Eradication and EliminationEradication and Elimination
 Termination of all transmission of infection by theTermination of all transmission of infection by the
extermination of theextermination of the infectious agentinfectious agent throughthrough
surveillance and containment. Eradication is ansurveillance and containment. Eradication is an
absolute process, anabsolute process, an ““all or noneall or none”” phenomenon,phenomenon,
restricted to termination of infection from therestricted to termination of infection from the
whole world.whole world.
 The termThe term elimination is sometimes used toelimination is sometimes used to
describe eradication of a disease from a largedescribe eradication of a disease from a large
geographic region. Disease which are amenablegeographic region. Disease which are amenable
to elimination in the meantime are polio, measlesto elimination in the meantime are polio, measles
and diphtheria.and diphtheria.

28. Reproductive rate of infectionReproductive rate of infection::
 Reproductive rate of infection: potentialReproductive rate of infection: potential
for an infectious disease to spread.for an infectious disease to spread.
Influential factors include the probability ofInfluential factors include the probability of
transmission between an infected and atransmission between an infected and a
susceptible individual; frequency ofsusceptible individual; frequency of
population contact; duration of infection;population contact; duration of infection;
virulence of the organism and populationvirulence of the organism and population
immune proportionimmune proportion ..

29. Dynamics of disease TransmissionDynamics of disease Transmission
(Chain of Infection)(Chain of Infection)
Source or Reservoir Modes of transmission Susceptible host
I II III

30. (I): Source or Reservoir(I): Source or Reservoir
 The starting point for the occurrence of aThe starting point for the occurrence of a
communicable disease is the existence of acommunicable disease is the existence of a
reservoir or source of infection.reservoir or source of infection.
 The source of infection is defined asThe source of infection is defined as ““the person,the person,
animal, object or substance from which ananimal, object or substance from which an
infectious agent passes or is disseminated to theinfectious agent passes or is disseminated to the
hosthost (immediate source).(immediate source). The reservoir isThe reservoir is ““anyany
person, animal, arthropod, plant, soil, orperson, animal, arthropod, plant, soil, or
substance, or a combination of these, in which ansubstance, or a combination of these, in which an
infectious agent normally lives and multiplies, oninfectious agent normally lives and multiplies, on
which it depends primarily for survival, and wherewhich it depends primarily for survival, and where
it reproduces itself in such a manner that it canit reproduces itself in such a manner that it can
be transmitted to a susceptible host. It is thebe transmitted to a susceptible host. It is the
natural habitat of the infectious agent.natural habitat of the infectious agent.””

31. Types of reservoirsTypes of reservoirs
Reservoir
Human
reservoir
Animal
reservoir
Non-living
reservoir

32. Human reservoirHuman reservoir
Human reservoir
cases carriers
According to spectrum of disease:
•Clinical cases
(mild/severe-typical/atypical)
•Sub-clinical cases
•Latent infection cases
•Primary case
•Index case
•Secondary cases
Type:
•Incubatory
•Convalescent
•healthy
Duration:
•Temporar
y
•Chronic
Portal of exit:
•Urinary
•Intestinal
•Respiratory
•others

33. CasesCases
 A case is defined asA case is defined as ““a person in thea person in the
population or study group identifiedpopulation or study group identified
as having the particular disease,as having the particular disease,
health disorder, or condition underhealth disorder, or condition under
investigationinvestigation””

34. CarriersCarriers
 It occurs either due to inadequate treatment or immuneIt occurs either due to inadequate treatment or immune
response, the disease agent is not completely eliminated,response, the disease agent is not completely eliminated,
leading to a carrier state.leading to a carrier state.
 It isIt is ““an infected person or animal that harbors a specifican infected person or animal that harbors a specific
infectious agent in the absence of discernible (visible)infectious agent in the absence of discernible (visible)
clinical disease and serves as a potential source ofclinical disease and serves as a potential source of
infection to others.infection to others.

Three elements have to occur toThree elements have to occur to form a carrier state:form a carrier state:
1.1. The presence in the body of the disease agent.The presence in the body of the disease agent.
2.2. The absence of recognizable symptoms and signs ofThe absence of recognizable symptoms and signs of
disease.disease.
3.3. The shedding of disease agent in the discharge orThe shedding of disease agent in the discharge or
excretions.excretions.

35. Animal reservoirsAnimal reservoirs
 Zoonosis is an infection that isZoonosis is an infection that is
transmissible under naturaltransmissible under natural
conditions from vertebrate animalsconditions from vertebrate animals
to man, e.g. rabies, plague, bovineto man, e.g. rabies, plague, bovine
tuberculosistuberculosis……....
 There are over a 100 zoonoticThere are over a 100 zoonotic
diseases that can be conveyed fromdiseases that can be conveyed from
animal to man.animal to man.

36. Reservoir in non-living thingsReservoir in non-living things
 Soil and inanimate matter can alsoSoil and inanimate matter can also
act as reservoir of infection.act as reservoir of infection.
 For example, soil may harbor agentsFor example, soil may harbor agents
that causes tetanus, anthrax andthat causes tetanus, anthrax and
coccidiodomycosis.coccidiodomycosis.

37. (II): Modes of transmission(II): Modes of transmission
Mode of transmission
Direct
transmission
Indirect
transmission
Direct
contact
Droplet infection
Contact with soil
Inoculation into skin or mucosa
Trans-placental (vertical)
Vehicle-borne
•Vector-borne:
•Mechanical
•biological
Air-borne
Fomite-born
Unclean hands
and fingers
propagative
Cyclo-prop.
Cyclo-develop.

38. (III): Susceptible host(III): Susceptible host
 An infectious agent seeks a susceptibleAn infectious agent seeks a susceptible
host aiminghost aiming ““successful parasitismsuccessful parasitism””..
 Four stages are required for successfulFour stages are required for successful
parasitism:parasitism:
1.1. Portal of entryPortal of entry
2.2. Site of election inside the bodySite of election inside the body
3.3. Portal of exitPortal of exit
4.4. Survival in external environmentSurvival in external environment

39. Virulence and Case Fatality RateVirulence and Case Fatality Rate
 VirulenceVirulence: is the degree of pathogenicity; the: is the degree of pathogenicity; the
disease evoking power of a micro-organism in adisease evoking power of a micro-organism in a
given host. Numerically expressed as the ratio ofgiven host. Numerically expressed as the ratio of
the number of cases of overt infection to the totalthe number of cases of overt infection to the total
number infected, as determined bynumber infected, as determined by
immunoassay. When death is the only criterion ofimmunoassay. When death is the only criterion of
severity, this is the case fatality rate.severity, this is the case fatality rate.
 Case fatality rate for infectious diseases:Case fatality rate for infectious diseases: isis
the proportion of infected individuals who die ofthe proportion of infected individuals who die of
the infection. This is a function of the severity ofthe infection. This is a function of the severity of
the infection and is heavily influenced by howthe infection and is heavily influenced by how
many mild cases are not diagnosed.many mild cases are not diagnosed.

40. Serial interval and Infectious periodSerial interval and Infectious period
 Serial interval: (the gap in time betweenSerial interval: (the gap in time between
the onset of the primary and thethe onset of the primary and the
secondary cases) the interval betweensecondary cases) the interval between
receipt of infection and maximal infectivityreceipt of infection and maximal infectivity
of the host (also called generation time).of the host (also called generation time).
 Infectious (communicable) period: lengthInfectious (communicable) period: length
of time a person can transmit diseaseof time a person can transmit disease
(sheds the infectious agent).(sheds the infectious agent).

41. Incubation and Latent periodsIncubation and Latent periods
 Incubation period: time from exposure toIncubation period: time from exposure to
development of disease. In other words,development of disease. In other words,
the time interval between invasion by anthe time interval between invasion by an
infectious agent and the appearance of theinfectious agent and the appearance of the
first sign or symptom of the disease infirst sign or symptom of the disease in
question.question.
 Latent period: the period betweenLatent period: the period between
exposure and the onset of infectiousnessexposure and the onset of infectiousness
(this may be shorter or longer than the(this may be shorter or longer than the
incubation period).incubation period).

42. Transmission Probability RatioTransmission Probability Ratio
(TPR(TPR))
TPR is a measure of risk transmissionTPR is a measure of risk transmission
from infected to susceptible individualsfrom infected to susceptible individuals
during a contact.during a contact.
TPR of differing types of contacts,TPR of differing types of contacts,
infectious agents, infection routes andinfectious agents, infection routes and
strains can be calculated.strains can be calculated.
There are 4 types of transmissionThere are 4 types of transmission
probabilities.probabilities.

43. TPR (contTPR (cont.).)
Transmission probabilities:Transmission probabilities:
 p00: tp from unvaccinated infective top00: tp from unvaccinated infective to
unvaccinated susceptibleunvaccinated susceptible
 p01: tp from vaccinated infective top01: tp from vaccinated infective to
unvaccinated susceptibleunvaccinated susceptible
 p10: tp from unvaccinated infective top10: tp from unvaccinated infective to
vaccinated susceptiblevaccinated susceptible
 p11: tp from vaccinated infective top11: tp from vaccinated infective to
vaccinated susceptiblevaccinated susceptible

44. TPR (contTPR (cont.).)
 To estimate the effect of a vaccine inTo estimate the effect of a vaccine in
reducing susceptibility, compare the ratioreducing susceptibility, compare the ratio
of p10 to p00.of p10 to p00.
 To estimate the effect of a vaccine inTo estimate the effect of a vaccine in
reducing infectiousness, compare the ratioreducing infectiousness, compare the ratio
of p01 to p00.of p01 to p00.
 To estimate the combined effect of aTo estimate the combined effect of a
vaccine, compare the ratio of p11 to p00.vaccine, compare the ratio of p11 to p00.

45. THANK YOU
THANK YOU