This document discusses key concepts related to disease transmission including: 1. The epidemiologic triad of an agent, host, and environment being required for disease transmission. 2. Various host, agent, and environmental factors that influence transmission risk. 3. Common modes of transmission like direct contact or indirect transmission through vehicles or vectors. 4. Key epidemiological terms like outbreak, epidemic, pandemic, and the differences between clinical and subclinical disease.

1. The Dynamics of Disease
Transmission
Second session
Dr. Amita Kashyap
Sr. Prof. Community Medicine
S.M.S. Med. College, Jaipur

2. • I keep six honest serving-men
• (They taught me all I knew);
• Their names are What and Why and
When
• And How and Where and Who.
• —Rudyard Kipling1 (1865–1936)

3. Learning Objectives
• To introduce concepts related to disease
transmission using the epidemiologic approach
to communicable diseases as a model.
• To define important terms related to the
occurrence of disease in a population.
• To calculate an attack rate and illustrate how it
may be used to measure person-to-person
transmission of a disease.
• To describe the steps in an outbreak
investigation and introduce how cross-
tabulation may be used to identify the source.

4. The epidemiologic triad – Disease
Transmission requirements
• Agent
• Reservoir for
agent
• Portal of exit
• Suitable mode
of
transmission
• Portal of entry
• Susceptible
host

5. Host
Characteristics
Types of Agents
and Examples
Environmental
Factors
1.Age
2.Sex
3.Race
4.Religion
5.Customs
6.Occupation
7.Genetic profile
8.Marital status
9.Family
background
10.Previous
diseases
11.Immune status
1.Biologic
2.Bacteria, viruses
3.Chemical
4.Poison, alcohol,
smoke
5.Physical
6.Trauma,
radiation, fire
7.Nutritional
8.Lack, excess
1.Temperature
2.Humidity
3.Altitude
4.Crowding
5.Housing
6.Neighborhood
7.Water
8.Milk
9.Food
10.Radiation
11.Air pollution
12.Noise
Factors That May Be Associated with
Increased Risk of Human Disease

6. Chain of Transmission
6
Eyes
Nose
Mouth
Skin
Urinary
tract
Genital
tract
Susceptible

7. Body surfaces as sites of microbial infection and shedding
Infections can be acquired through more than one route
The same routes also serve as points of entry
for noninfectious disease-causing agents

8. Modes of Transmission
1. Direct
a. Person-to-person contact
2. Indirect
a. Common vehicle
(1) Single exposure
(2) Multiple exposures
(3) Continuous exposure
b. Vector

9. Modes of Spread
Direct Transmission
 Contact
Ensures certainty
 Larger dose,
 Less chances of organism dying outside human host
 Airborne
 Droplet (3-5 microns), Distance 1-2 feet
 Droplet Nuclei (< 3 microns) Distance 3-5 feet
 Infected dust
 Transplacental - 9

10. A classic photograph showing
droplet dispersal after a sneeze
An infected individual
can transmit influenza
or the common cold to a
score of others in the
course of an innocent
hour in a crowded room.
A venereal infection
also must spread
progressively from
person to person
if it is to maintain itself
in nature, but it would
be a formidable task
to transmit venereal
infection on such a scale!!

12. Indirect
1. Through vehicles
Water
Food
Milk
Blood and plasma
Organs
 Cases confined to exposed population
 Large Geographic spread
 Cases start disappearing with vehicle control
 Common source is traceable
 The Potential to spread & produce outbreak
depends on characteristics of the organism &
its route of transmission 12
1. Single Exposure
2. Multiple Exposure
3. Continuous Exposure

13. 2. Through Vectors
Mechanical – Housefly
Biological
Propogative (Multiply numbers)
e.g. Microfilaria in mosquito
Cyclo- developmental (Change in form)
e.g. Malarial parasite in mosquito
Cyclo- propogative (change in form &
number too)
e.g. Plague bacilli in Rats
13

14. Herd Immunity
• Reduced probability of an individual becoming
infected when it is part of a vaccinated population.
• Individuals who are immune to a disease act as a
barrier in the spread of disease.
• The proportion of immune individuals in a
population above which a disease may no longer
persist is the herd immunity threshold.
• This concept is important in disease elimination or
eradication programs e.g. elimination can be
achieved without necessarily vaccinating the
entire population.

15. Herd Immunity

16. For Herd Immunity
Necessary conditions:
–Disease agent must be restricted to a
single host species within which
transmission occurs
–Transmission must be direct from one
member of the host species to another
(No other Reservoir)
–Infections must induce solid immunity
16

17. Herd Immunity
• Works when:
–Probability of an infected person
encountering every other individual in the
population is the same (random mixing)
• Does NOT work when:
–An infected person interacts only with
people who are susceptible
17

18. Types of Herd Immunity
1. Innate (Inherent) Herd Immunity:
Genetically determined physiological changes
with respect to antibody production or other
defense mechanism in a herd.
2. Acquired Herd Immunity:
where a sufficient number of its members
have actually been exposed naturally or
artificially to infectious agents during their
lifespan.

19. Clinical and Subclinical Disease
The “iceberg” concept of infectious diseases

20. Distribution of clinical severity for three classes of
infections

21. Nonclinical (Inapparent) Disease
Nonclinical disease may include the following:
1. Preclinical Disease- Disease that is not yet
clinically apparent but is destined to progress
to clinical disease.
2. Subclinical Disease- Disease that is not
clinically apparent and is not destined to
become clinically apparent. This type of
disease is often diagnosed by serologic
(antibody) response or culture of the organism

22. Nonclinical (Inapparent) Disease
3. Persistent (Chronic) Disease- A person fails to “shake
off” the infection, and it persists for years, at times for
life.
Post-polio syndrome in adult life. These have thus
become cases of clinical disease, albeit somewhat
different from the initial illness.
4. Latent Disease- An infection with no active
multiplication of the agent, as when viral nucleic acid is
incorporated into the nucleus of a cell as a provirus.
In contrast to persistent infection, only the genetic
message is present in the host, not the viable
organism.

23. Carrier Status
• An individual who harbors the organism but is
not infected as measured by serologic studies (no
evidence of an antibody response) or by evidence
of clinical illness.
• This person can still infect others, although the
infectivity is often lower than with other infections.
• May be of limited duration or may be chronic,
lasting for months or years.
– Typhoid Mary, who carried Salmonella typhi and died in
1938. Over a period of many years, she worked as a cook
in the New York City area, moving from household to
household under different names. She was considered to
have caused at least 10 typhoid fever outbreaks that
included 51 cases and 3 deaths.

24. Level of Disease
• Sporadic - a disease that occurs infrequently and
irregularly.
• Endemic - the habitual presence of a disease
within a given geographic area
• Hyper endemic - to persistent, high levels of
disease occurrence.
• Epidemic - the occurrence in a community or
region of a group of illnesses of similar nature,
clearly in excess of normal expectancy

25. Level of Disease Continued
• Outbreak carries the same definition of epidemic,
but is often used for a more limited geographic
area.
• Cluster refers to an aggregation of cases grouped
in place and time that are suspected to be greater
than the number expected, even though the
expected number may not be known.
• Pandemic refers to an epidemic that has spread
over several countries or continents, usually
affecting a large number of people

26. Endemic, Epidemic, and Pandemic

27. Level of Disease

28. Epidemic
• A recent increase in amount or virulence of
the agent,
• The recent introduction of the agent into a
setting where it has not been before (All
Susceptible),
• An enhanced mode of transmission,
• A change in the susceptibility of the host
• Factors that increase host exposure or involve
introduction through new portals of entry.

29. Exercise
• For each of the following situations, identify whether it reflects:
A. Sporadic disease
B. Endemic disease
C. Hyper-endemic disease
D. Pandemic disease
E. Epidemic disease
• ____ 22 cases of legionellosis occurred within 3 weeks among residents of
a particular neighborhood (usual occurrence is 0 or 1 per year)
• ____ Average annual incidence was 364 cases of pulmonary tuberculosis
per 100,000 population in one area, compared with national average of
134 cases per 100,000 population
• ____ Over 20 million people worldwide died from influenza in 1918–1919
• ____ Single case of histoplasmosis was diagnosed in a community
• ____ About 60 cases of gonorrhea are usually reported in this region per
week, slightly less than the national average

30. Types Of Epidemic
As per their manner of spread through :
1. Common-source
a) Point
b) Continuous
c) Intermittent
2. Propagated
3. Mixed
4. Other

31. 31
Common Source
Cases occur suddenly after minimum incubation time
All cases occur within one incubation period
Outbreak stops unless secondary spread
Curves have steep upslope, more gradual down slope
Common Point Source Transmission

32. Common Point Source Transmission
IP 15-50 Days (Av. 30 Days)

33. Continuous common source epidemics –
Common Source - Broad Street pump.
Cholera has an incubation period of 1-3 days

34. 34
• In this case, there are several peaks, and the
incubation period cannot be identified.
Intermittent Exposure

35. Propagated (or progressive source) epidemic

36. Propagated Outbreaks
36

37. Endemic, Epidemic, and Pandemic

38. Endemic, Epidemic, and Pandemic
• Endemic is defined as the habitual presence of
a disease within a given geographic area
• Epidemic is defined as the occurrence in a
community or region of a group of illnesses of
similar nature, clearly in excess of normal
expectancy, and derived from a common or
from a propagated source.
• Pandemic refers to a worldwide epidemic.

39. Daytime (10:30 am) photographs of the Great Smog’s toxic pollution
In December 1952, from 6 to 9th Dec. the fog was so thick
that at times, people could not see their own hands and feet.
More than 4,000 deaths were attributed to the fog
Led to :-
legislation,
including the
Clean Air Acts
of 1956 and
1968

40. Disease Outbreaks
• If a food becomes contaminated with a
microorganism and an outbreak occurs in the
group of people who have eaten the food, it is
called a common-vehicle exposure; it can be -
– Single Exposure or
– Multiple Exposure or
– Periodic / intermittent or
– Continuous exposure

41. 41
Point Source Outbreaks
• All exposed at one time
• Cases occur suddenly after minimum
incubation time
• All cases occur within one incubation
period
• Outbreak stops unless secondary spread
• Curves have steep upslope, more gradual
down slope

42. 42
• This is the most common form of transmission in
food-borne disease, in which a large population is
exposed for a short period of time.
Point Source Transmission

43. 43
Continuous Common Source
• May begin suddenly or gradually
• Cases do not disappear because of
secondary exposure
• Curves have gradual or steep
upslope, plateau trickling down
slope, and may repeat

44. 44
• In this case, there are several peaks, and the
incubation period cannot be identified.
Continuing Common Source or
Intermittent Exposure

45. 45

46. Immunity and Susceptibility
• The amount of disease in a population depends
on a balance between the number of susceptible
people in that population and the number of
people who immune, and therefore not at risk.
• people may be immune because they have had
the disease previously or because they have been
immunized OR on a genetic basis.
• When balance moves toward susceptibility, the
likelihood of an outbreak increases.

47. Herd Immunity
• If a large percentage of the population is
immune, the entire population is likely to be
protected, not just those who are immune.
• For herd immunity to exist :-
– The disease agent must be restricted to a single host
species within which transmission occurs,
– Transmission must be relatively direct from one
member of the host species to another.
– No other means of transmission are available.
– Infections must induce solid immunity.
– If the probability of an infected person encountering
every other individual in the population (random
mixing) is the same

48. Effect of herd immunity, United States, 1958–1961

49. Incubation Period
• The incubation period is defined as the interval
from receipt of infection to the time of onset of
clinical illness.
• Different diseases have different incubation
periods. A precise incubation period does not
exist for a given disease; rather, a range of
incubation periods is characteristic for that
disease.
• In general, the length of the incubation period is
characteristic of the infective organism

50. Incubation periods of viral diseases

51. Incubation periods for 191 delegates affected by
a Salmonella typhimurium outbreak

52. Number of cases plotted against time and against the
logarithm of time