This document discusses various concepts related to the manifestations of disease in individuals and communities. It covers topics such as the spectrum of disease, infectious agents, chains of disease transmission, routes of spread, incubation periods, and carrier states. The key points are: 1. The spectrum of disease reflects the sequence of illness manifestations in a host responding to an infectious agent, ranging from subclinical to severe illness. 2. Chains of disease transmission involve an infectious agent, reservoir, portals of entry/exit, modes of spread, and susceptible hosts. 3. Routes of spread can be direct through contact or indirect through vehicles, vectors, air, fomites, or hands. Incubation periods determine the

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COMMUNITY MEDICINE
MANIFESTATIONS OF DISEASE IN INDIVIDUALS AND THE COMMUNITY
Dr.Gahayr
Lec 5
15/11/2022

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1. BASIC CONCEPTS
1. Infection:
Entry and development/multiplication of disease causing micro
organisms in man or animals. May be contained or progress to
disease depending on the immunity
2. Latent infection: the organism is not being shed, but may get
reactivated after some time e.g. HSV (herpes simplex virus),TB
3. Pathogenicity: is the probability of developing symptoms or disease
after becoming infected and is host and agent interaction

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BASIC CONCEPTS…
4. Virulence is the ability of the pathogen to infect/kill or damage a
host.
5. Infectivity is the ability of a pathogen to establish an infection.
6. Induction : time to disease initiation
7. Incubation:– time to symptoms (infectious disease)

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BASIC CONCEPTS…
8.Nosocomial or hospital acquired infection- contract infection during
his/her stay in hospital e.g. UTI (catheter), AIDS (unsafe injection).
9. Cross infection- contracted from fellow patients/ hospital
environment
10. Iatrogenic diseases (physician induced)-as a result of treatment or
investigation procedures in the hospital e.g. allergy and adverse
reaction to drugs
11. Infectious disease- disease resulting from an infection e.g. leprosy
and malaria

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BASIC CONCEPTS…
12.Infestation-lodgment,development,and reproduction of arthropods
on the surface of the body e.g. itch mite. Also presence of parasitic
worms in the gut e.g. ascaris and ancylostoma.
13. Contamination- presence of infectious agent on body surface, in/on
cloths, beddings, toys, surgical instruments, dressings and other
inanimate articles like water, milk or food.
14. Pollution- presence of offensive but not necessarily infectious agent
in the environment e.g. water, soil pollution

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BASIC CONCEPTS…
15. Contagious- disease transmitted through physical contact e.g.
scabies, STI and leprosy.
16. Communicable disease- disease caused by specific micro organism
which is transmitted from person to person, animal to animal, animal
to man and environment to man either directly or indirectly e.g.
cholera, typhoid, STD etc.

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2.SPECTRUM OF DISEASE
• Spectrum of diseases- is the sequence of events that occur in human
from the time of exposure to etiological agent to death.
• In infectious diseases, this spectrum is known as gradient of infection-
which reflects the sequence of manifestations of illness in the host
reflecting his response to the infectious agent.

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SPECTRUM OF DISEASE…
• Gradient of infection is different types of host response to an
infection:
=>In apparent or sub clinical=> presence of an infection in a level below
the threshold of clinical symptoms=> mild illness=> moderate
illness=> severe/recovery or death

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SPECTRUM OF DISEASE…
In apparent infection- is the presence of an infection below the
threshold of clinical symptoms. The importance are:
1. May exceed clinical cases in number
2. May be more infectious than overt cases
3.Play role in determining the size of the susceptible, since they may
induce protective antibody e.g. polio

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3. INFECTIOUS AGENT
• Has its own:
- Life cycle
- Immunology
- Ecology
- Evolution
- Molecular biology
• All, except life cycle, imply emerging and re- emerging infection and
resistance

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4. CHAINS OF COMMUNICABLE DISEASE TRANSMISSION
The six links in the chain are:
I. Infectious agent/etiology- is an organism that is capable of producing
infection or infectious disease. The biological properties of the agent may
play a major role in its epidemiology. In order to survive the agent must
be able to:
• Multiply
• Emerge from the host
• Reach the new host
• Infect the new host

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CHAINS OF COMMUNICABLE…
II. Reservoir- is the person or animal, arthropod, plant, soil or substance in
which an infectious agent can thrive and from where it can be transmitted
to another host.
III. The Portal of Entry.
IV. Portal of exit- Way in which the agent gets excreted from the host.

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CHAINS OF COMMUNICABLE…
V. Mode of spread- means of spread from reservoir to susceptible
host
• Infectious agent thrive in person, animal, arthropod, plant, soil and
substance, and spread to susceptible through source.
• The material containing an infectious agent through which the actual
transfer of the agent from the reservoir to susceptible host takes
place e.g. in pulmonary TB, sputum is the source
• Reservoir: may be person, animal, arthropod, plant, soil or substance

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CHAINS OF COMMUNICABLE…
• Human reservoir may be overt case, sub clinical case or carrier
• Animal reservoir e.g. rabies (dogs), Japanese encephalitis (birds, pigs)
• Soil: Clostridium tetanus, B. anthraces
• Zoonotic- communicable disease transmitted from vertebrate animals
to man under natural conditions e.g. rabies, plague, anthrax,
salmonellas, hydatid cyst, bovine TB, KFD and monkey pox.
VI. The Susceptible Host

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CHAINS OF COMMUNICABLE…
• Man to vertebrate: zoonanthroposis e.g. Tb
• Vertebrate to man: anthropozoonosis e.g. rabies
• Two way: amphixenosis e.g. T. cruzi, S.japonicum
• Epizootic (epidemic in animals) e.g. anthrax, brucellosis, rabies, Q-
fever. encephalitis and EEE
• Enzootic (endemic in animals) : e.g. anthrax, rabies, brucellosis and
endemic typhus

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5. ROUTES OF SPREAD
1.Direct
A. Direct contact: no intervening agency from reservoir/source to a
susceptible host e.g. STD, AIDS and scabies.
B. Droplet infection: direct projection of droplets spray of saliva or
nasopharyngeal secretion 30-60cm. into in to hosts nose or mouth
by acts like sneezing coughing or talking e.g. common cold,
diphtheria, TB, meningococcal meningitis. Risks are over crowding
and lack of ventilation.

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ROUTES OF SPREAD…
C. Contact with soil: tetanus, mycosis
D. Bite of animal: rabies
E. Placental/vertical: syphilis, AIDS, German measles, drugs.
2.Indirect
A. Vehicle: water, food, milk, milk products, ice, blood, serum,
plasma or other biological products.

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ROUTES OF SPREAD…
B. Vector:
1. biological: development or multiplication of disease causing
organism before it is being transmitted to susceptible host.
2. mechanical:
- Invertebrate: flies, mosquitoes, fleas, lice bugs, ticks, mites and
Cyclops
- Vertebrate: mice, rats and bats

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ROUTES OF SPREAD…
- Man-arthropod: malaria
- Mammal or bird-arthropod: plague, Japanese encephalitis
- Man- two intermediate hosts: fish tapeworm.
C. Air (droplet nuclei): TB, influenza, chicken pox, measles and Q-fever/dust=TB,
streptococcal infection.
D. Fomites: inanimate things other than food, water or milk
e.g. cloths (soiled), towels, handkerchiefs, cups, spoons, toys, pencils, door
handle, lavatory chains, syringes and surgical dressings diphtheria, typhoid,
bacillary dysentery, eye and skin infections

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ROUTES OF SPREAD…
E. Hands and fingers:
Risks factors are lack of hygiene
Pathogens from skin, nose, feces are transmitted to susceptible host
e.g. typhoid, dysentery, infective hepatitis, intestinal worms, staphylococcal and
streptococcal infections.
V. Portal of entry- route through which infectious agent enters the host.

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ROUTES OF SPREAD…
VI. Susceptible host
• Host- a person or animal that harbors an infectious agent under natural
conditions.
• Obligate host-the only host for specific organism e.g. man in typhoid.
• Definitive or primary host- one in which parasite attains maturity (man in ascaris)
or passes its sexual stage (mosquito in malaria).
• Intermediate host- one in which larval or asexual stage of parasite is spent (man
in malaria)

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6. INCUBATION PERIOD
• Is the time interval between entry and development of signs and
symptoms of disease. It is the time interval between the entry of
pathogenic micro organism and sufficient multiplication to cause
pathology that will produce signs and symptoms of disease.

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INCUBATION PERIOD…
• Incubation period (in non infectious diseases): is equivalent to
induction + latent period.
• Chicken pox- latent period is less than incubation period, therefore
quarantine has no use
• Malaria has incubation period of 14 days, stages infective to mosquito
develop 10 days after incubation period.
• HIV- short latent period. Two periods of latency:

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INCUBATION PERIOD…
1. Latent period for infectiousness
2. Latent period for seroconversion
- Virulence, load and host response determine incubation period
- Short incubation period implies 2-3 days e.g. cholera, influenza
- Intermediate incubation period implies 10-21 days e.g. typhoid
- Long incubation periods imply several weeks- months/years e.g.
leprosy, TB and rabies

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USE OF KNOWLEDGE OF IP
• Trace source of infection and contacts
• Determine the period of surveillance/ quarantine
• Prevention of disease by using immunoglobulin and antiserum
• Determine an epidemic as point source or propagated/ exponential
• Assess prognosis of an infectious disease

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7. CARRIER STATE
• A condition in which a man or animal harbors and excretes infectious
agent without overt clinical signs and symptoms. Are at least three
types.
A. Based on the course of infection:
1. Incubatory carriers: in apparent infection
2. Convalescent carriers
3. Chronic carriers

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CARRIER STATE…
B. Based on the route of exit
1. Urinary carriers
2. Faecal carriers
3. Throat carriers
• Infectious carrier= recovered but infectious
• In apparent/silent case= successful infection that does not develop
symptoms- can be infectious

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CARRIER STATE…
• Epidemiologically, carriers are more important than overt clinical
cases because identification may require sophisticated investigation
but important to control transmission e.g. AIDS

29. Thank you for your attention
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