PUBLIC HEALTH

1. 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 and its
Level in a population.

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

3. Chain of Transmission Mode
5
Eyes
Nose
Mouth
Skin
Urinary
tract
Genital
tract

4. 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

5. 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 - 7

6. A Classic Picture showing Droplet Dispersal
after a Sneeze
1. Every Organism has
unique way for its
spread
2. The potential of Spread
and producing
outbreaks depends on
1. its rate of growth
2. the route of
transmission
from one person
to another

7. 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 9
1. Single Exposure
2. Multiple Exposure
3. Continuous Exposure

8. 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
10

9. 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.

10. Herd Immunity

11. 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
13

12. 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
14

13. 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.

14. 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

15. 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

16. Endemic, Epidemic, and Pandemic

17. Level of Disease

18. 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.

19. 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

20. 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

21. 23
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

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

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

24. 26
• In this case, there are several peaks, and the
incubation period cannot be identified.
Intermittent Exposure

25. Propagated (or progressive source) epidemic

26. Propagated Outbreaks
28

27. 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

28. 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

29. 31
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

30. 32
• 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

31. 33
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

32. 34
• In this case, there are several peaks, and the
incubation period cannot be identified.
Continuing Common Source or
Intermittent Exposure

33. 35