This document defines key terms related to communicable diseases and their transmission. It begins by defining infection, contamination, pollution, infestation, infectious disease, communicable disease, and contagious disease. It then describes the different modes of disease transmission, including direct transmission through contact, droplets, soil, inoculation, and transplacentally. Indirect transmission occurs through vehicles like water, food, and air; or vectors like arthropods. Diseases can be transmitted via fomites, dust, or unclean hands and fingers. Communicable diseases are spread through these various direct and indirect modes of transmission between humans, animals, and the environment.

1. Introduction
to
Communicable diseases
Dr.Rajkumar Patil
Professor, Community Medicine, MGMCRI 1

2. Learning objectives
At the end of the class student should be able to:
• Define communicable disease
• Describe the basic differences between
communicable and non-communicable diseases
• Enumerate the modes of transmission of
communicable disease
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3. Definitions
• Infection
• Contamination and Pollution
• Infestation
• Communicable disease
• Infectious disease
• Contagious disease
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4. INFECTION
• “The entry and development or multiplication
of an infectious agent in the body of man or animals”
• An infection does not always cause illness.
• Levels:
a. Colonization(e.g.- S. aureus in skin & normal
nasopharynx)
b. Subclinical or inapparent infection (e.g.- polio)
c. Latent infection (e.g.- virus of herpes simplex)
d. Manifest or clinical infection.
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5. CONTAMINATION AND POLLUTION
• “The presence of an infectious agent on a body surface;
also on or in clothes, beddings, toys, surgical
instruments or dressings, or other inanimate articles or
substances including water, milk and food”
• Contamination on a body surface does not imply a
carrier state.
• Pollution is distinct from it & implies the presence of
offensive, but not necessarily infectious matter.
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6. INFESTATION
• “For persons or animals the lodgement, development and
reproduction of arthropods on the surface of the body or
in the clothing, e.g., lice, itch mite”
• Invasion of the gut by parasitic worms, e.g., ascariasis.
• Infested premises: Those which harbour or give shelter to
animal forms, especially arthropods and rodents.
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7. INFECTIOUS DISEASE
“A clinically manifest disease of man or animals
resulting from an infection”
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8. COMMUNICABLE DISEASE
“An illness due to a specific infectious agent or
its toxic products capable of being directly or
indirectly transmitted from man to man, animal to
animal, or from the environment (through air, dust,
soil, water, food, etc.) to man or animal”.
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9. Communicable diseases
• Respiratory Infections
• Intestinal Infections
• Arthropod borne infections
• Zoonotic diseases(Rabies,Leptospirosis,JE etc.)
• Surface infections(Trachoma,Leporsy,STD etc.)
• Hospital acquired infection
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10. What are the basic differences
between Communicable and
Non-communicable diseases?
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11. CONTAGIOUS DISEASE
“A disease that is transmitted through contact”.
Examples: scabies, trachoma, STD and leprosy.
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12. MODES OF
DISEASE TRANSMISSION
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13. 13

14. A) DIRECT TRANSMISSION
• Direct contact
• Droplet infection
• Contact with soil
• Inoculation-skin or mucosa
• Transplacental (vertical)
B) INDIRECT TRANSMISSION
• Vehicle-borne
• Vector-borne
a. Mechanical b. Biological
• Air-borne
 Droplet nuclei
 Dust
• Fomite -borne
• Unclean hands & fingers
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15. DIRECT TRANSMISSION
(1) Direct contact
• Skin - skin, mucosa - mucosa, or mucosa-skin of the same or
another person.
• From the reservoir or source to a susceptible individual, without an
intermediate agency
• Examples: skin-to-skin contact as by touching, kissing or sexual
intercourse
• Diseases transmitted - STD and AIDS, leprosy, leptospirosis, skin and
eye infections.
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16. (2) Droplet infection
• Direct projection of a spray of droplets of
saliva & naso-pharyngeal secretions during coughing,
sneezing, or speaking, talking.
• Examples: Resp. infection, eruptive fevers, TB,
meningococcal meningitis, etc.
• Upon the conjunctiva, oro-respiratory mucosa
• Particles: 5 micrometers & less reach the alveoli.
• Limited to distance of 1-2 ft between source & host. 16

17. (3) Contact with soil
• Direct exposure of susceptible tissue to the disease
agent in soil, compost or decaying vegetable
matter.
• Examples: hookworm larvae, tetanus, mycosis etc.
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18. Hookworm
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19. (4) Inoculation into skin or mucosa
• Rabies virus by dog bite, Hep. B virus through contaminated
needles and syringes etc.
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20. (5) Transplacental (or vertical) transmission
• TORCH agents, varicella virus, syphilis,
Hep-B, Coxsackie B and AIDS.
• Some of the non-living agents (e.g., thalidomide,
diethylstilbestrol) can also be transmitted vertically.
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22. Indirect transmission
• Traditional 5 F's - "flies, fingers, fomites, food
and fluid".
• Infectious agent survives outside retaining its
basic properties of pathogenesis & virulence.
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•Infectivity (ability to infect)- (number infected / number susceptible) x 100
•Pathogenicity (ability to cause disease)-(number with clinical disease /
number infected) x 100
•Virulence (ability to cause death) -(number of deaths / number with
disease) x 100
All are dependent on host factors

23. Fomites
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24. 24
Vehicle-borne
• water,
• food,
• raw vegetables and fruits,
• milk,
• ice,
• blood

25. Food poisoning example:
In order to grow and multiply germs need
HeatFoodMoistureTime
Remember it like this
Too Many Flies Waiting

26. These bacterial cells are beginning to divide into two

27. After 10 minutes
After 20 minutes
After 30 minutes
After 40 minutes

28. Time : 9.30 Bacteria : 0
Time : 9.40
Time : 9.50
Time : 10.00
Bacteria : 12,000
Bacteria : 24,000
Bacteria : 48,000
Time : 10.10
Time : 10.20
Time : 10.30
Time : 10.40
Time : 10.50
Bacteria : 96,000
Bacteria : 192,000
Bacteria : 384,000
Bacteria : 768,000
Bacteria : 1.5 million
From 0 to 1,536,000 in
only 80 minutes !!!!!!
Knife
contaminated
by blood
cooking chicken to a core temperature
of 75°C should kill most of the bacteria

29. Epidemiological features of vehicle transmission
• Heavy dose of contamination – explosive outbreak
e.g., cholera & Hep-A epidemics
• When sec. cases occur, primary may be obscured
• Distance travelled may be great, e.g., food poisoning
• Its not always possible to isolate the infectious agent in the
vehicle, e.g., typhoid bacilli in contaminated water
• When the vehicle is controlled or withdrawn, the epidemic
subsides, e.g., epidemics of cholera
• The common source of infection is often traceable
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30. Vector-borne
• “vector is defined as an arthropod or any living
carrier (e.g., snail) that transports an infectious
agent to a susceptible individual”.
• Transmission -mechanical or biological.
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31. Epidemiological classification of
vector-borne diseases
I. By vector :
Invertebrate type : Arthropod vectors fall into seven orders largely
(1) Diptera - flies and mosquitoe
(2) Siphonaptera - fleas
(3) Orthoptera - cockroaches
(4) Anoplura - sucking lice
(5) Hemiptera - Bugs, including kissing bugs
(6) Acarina - ticks and mites
(7) Copepoda - cyclops
Vertebrate type: Mice, rodents, bats
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32. II. By transmission chain
a) Man and a non-vertebrate host
1. Man-arthropod-man (malaria)
2. Man-snail-man (schistosomiasis)
b) Man, another vertebrate host, and a non-vertebrate host
1. Mammal-arthropod-man (plague)
2. Bird-arthropod-man (encephalitis)
c) Man and 2 intermediate hosts
1. Man-cyclops-fish-man (fish tape worm)
2. Man-snail-fish-man (Clonorchis sinensis)
3. Man-snail-crab-man (Paragonimiasis) 32

33. III. By methods in which vectors transmit
agent:
• Biting
• Regurgitation
• Scratching
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IV. By methods in which vectors are involved in the
transmission and propagation of parasites:
(a) Mechanical transmission :
Crawling or flying arthropod through soiling of its feet or
proboscis; or by passage of organisms through its GIT and
passively excreted.
There is no development or multiplication of the infectious
agent on or within the vector.

35. (b) Biological transmission:
The infectious agent undergoing replication or development
or both in vector and requires an incubation period before
vector can transmit.
Three types:
(i) Propagative: The agent merely multiplies in vector, but no
change in form, e.g., plague bacilli in rat fleas
(ii) Cyclo-propagative : The agent changes in form and
number, e.g., malaria parasites in mosquito
(iii) Cyclo-developmental : The disease agent undergoes only
development but no multiplication, e.g., microfilaria in
mosquito.
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36. Airborne transmission
(1) Droplet nuclei
• Tiny particles (1-10 mic) represent dried residue of
droplets. Formed by droplets coughed or sneezed into
the air.
• Diseases spread : TB, influenza, chickenpox, measles, Q
fever, Resp. infections.
• Spread of toxic air pollutants including "smog" results in
air pollution epidemics.
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37. (2) Dust: Droplets expelled during talking, coughing or
sneezing, settle down & become part of the dust.
• Infectious agents (e.g., streptococci, other pathogenic bacteria,
viruses, spores) have been found in the dust of hospital wards
• Some (e.g., TB bacilli) survive in dust for considerable periods
• During the act of sweeping, dusting & bed-making, the dust is
released into the air.
• Transmission is most common in Hospital-acquired (nosocomial)
infection.
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38. Fomite-borne
• Inanimate articles/ substances other than water or
food contaminated by the infectious discharges from a
patient and capable of harbouring and transferring to
a healthy person.
• Example: soiled clothes, towels, linen, handkerchiefs,
cups, spoons, pencils, books, toys
• Diseases transmitted: diphtheria, typhoid fever,
bacillary dysentery, hepatitis A, eye and skin infections.
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39. Unclean hands and fingers
• Most common medium ,transferred to food
from the skin, nose, bowel, etc.
• Transmission: Directly (hand-to-mouth)&
Indirectly, e.g.- staph./strepto. infections,
typhoid fever.
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