Natural history of disease describes the typical course and progression of a disease from its earliest detectable stage through recovery, disability, or death in the absence of treatment or prevention. Some key aspects discussed include important terminology like infection, communicable disease, incubation period, and carrier. The document also discusses stages of disease progression like pre-pathogenesis, early pathogenesis, and late pathogenesis. It describes the host response to infection which can include resistance, asymptomatic response, symptomatic response, hypersensitivity, and immunity.

1. Natural History of Disease

2. Learning objectives
• What is natural history of disease
• Important terminology
• Stage of disease
• Incubation period
• Host response to infection
• Levels of prevention

3. Natural History of Disease
It is the way in which a disease evolves over
time from the earliest stage of its pre-
pathogenesis phase to its termination as
recovery, disability or death, in the absence
of treatment or prevention

4. Some important terminologies
• Infection: Entry, development and / or
multiplication of an infectious agent in a living
being
• Infectious disease - Disease due to an infection
e.g. tuberculosis of lungs detected by sputum
examination.
• Communicable disease – A disease which is due
to direct or indirect transmission of a micro-
organisms or its toxic products in between
human beings, animals and environment.

5. Some terminologies
• Infestation - Entry, lodgment, development
and reproduction of invertebrate vectors or
parasites on the body surfaces, clothing or
within the body cavities.
• Communicability - The capacity of or readiness
of direct or indirect transmission of an
infectious agent in between human beings
and animals from one who is infected to the
other who is not.

6. Some terminologies
• Endemic - Constant perineal presence of an
infectious agent or disease at a low level in a
delineated territory and population without
importation from outside and without
excessive fluctuations in diseases frequency.
• Epidemic - Occurrence of disease frequency in
a delimited geographic area and defined
population during a specific time period,
clearly in excess of normal expectation.

7. Some terminologies
• Pandemic - Simultaneous occurrence or
speedy propagation of epidemic in more than
one country or over a wide geographic area;
involving a large section of population.
• Sporadic - Occurrence of disease widely
separated in time and space over a large
geographic area and with no interrelationship
of any kind.

8. Some terminologies
• Zoonoses: Transmission of infection in
between human beings and vertebrate
animals.
• Epizootic: Epidemic in animals
• Source - A living or nonliving thing which
harbors the disease agent, which may survive
and multiply within it, but does not depend
for survival on it.: E.g. Contaminated water
• Reservoir: Agent depends on survival: man for
cholera

9. Some definitions
• Carrier - An infected living being that harbours
the agent of disease without any clinically
detectable disease and is capable of excreting
and transmitting the disease agent.
Types:
• Duration: Temporary
• Route of excretion: Urinary/ Respiratory/ fecal
• Healthy/ Contact/ Convalescent

10. Iceberg of Disease

11. Iceberg of Disease
• Floating tip of the iceberg represents what the
physician sees in the community-clinical cases
• The vast submerged portion of the iceberg
represents the hidden mass of disease- latent,
inapparent, presymptomatic and undiagnosed
cases and carriers in the community
• Water line represents the demarcation between
apparent and inapparent disease

12. Natural History of Disease

13. • Typhoid fever:
o Infecting organism enters the body through...
o Incubation period of 14 days
o Clinical manifestation: headache, remittent fever rise
to step ladder pattern, initial constipation followed by
pea soup tool.
o Second week: spleen palpable, rash appears
o Third: most of recover, intestinal hemorrhage,
perforation, peritonitis, mortality, Carrier

14. Stages in disease
Pre-pathogenesis
Pathogenesis: Early / Late
PRE-PATHOGENESIS
• Before onset of disease
• Pathogen not inside body
• Balanced state
• Presence of risk factors

15. Pathogenesis
• Entry of disease agent in body
• Agent starts multiplying/ developing
• Reaches target organ
• Overcomes host defenses
• Detectable host response
• Two phases

16. Incubation period
• Time from entry of organism inside the body
of host to first sign or symptom
• Time is required for:
Travel to site of selected organ
Multiplication
Development
Toxin production

17. Incubation period
• Specific for specific disease
• Has a range
Uses of Incubation Period
• Finding source of infection
• Calculating secondary attack rate
• Differential diagnosis (Food poisoning)
• Determining period of quarantine
• Declaring end of epidemic
• Severity of disease (Tetanus)

18. Early Pathogenesis
• No clinical signs symptoms
• Host response at cellular level only
• Host response detectable by laboratory tests
only

19. Late Pathogenesis
• Host response detectable by signs / symptoms
• Laboratory tests can also show host response
Outcome
- Full recovery
- Partial recovery with impairment/ disability
- Death

20. Host Response to Infection
Resistance
- Innate immunity
- Natural ability to
combat and avoid
infection
- Non-specific / Specific
Symbiosis
Entry of agent does not
lead to any host
response
Host and agent both
derive benefit
Commensalism
Similar to symbiosis but
Host does not get any
benefit

21. Host Response to Infection
Asymptomatic
response
- Agent enters host
- Does not evoke
clinical response
- Existence of agent
can be shown by
some tests
Symptomatic response
- Agent enters host
- Evoke clinical
response
- Existence of agent
can also be shown by
some tests

22. Host Response to Infection
Hypersensitivity (Allergy)
• Response on re-exposure
• First exposure is un-eventful: Sensitizes
• Second exposure leads to different and specific
clinical response
• Hypersensitivity = Injurious consequences
• Immediate type: Anaphylaxis, Arthus
Phenomenon, Serum Sickness
• Delayed type: e.g. TB

23. Host Response to Infection
Immunity
Response on exposure
First exposure evokes changes in the form
of antibody formation
Clinical response is no disease, milder
disease, modified disease

24. Epidemiological triad

25. Agent
Anything living or non-living the existence/
excess/ or lack of which is the necessary cause
of the disease
Physical: Heat, Cold
Chemical: Fluoride, Vitamins
Biological: Viruses, Bacteria, Protozoa,
Helminths

26. Host: Types
The one which harbors the disease agent
Definitive host is one in which sexual cycle of
development takes place (the female anopheles
for malarial parasite)
Intermediate host is one in which asexual cycle
of development takes place (human beings for
malarial parasite).

27. Host: Types
• Maintenance host When the microbe and the
host live symbiotically (monkey for yellow
fever).
• Incidental host accidentally affected, becomes
infected but cannot transmit the agent
regularly and usually functions as a dead end
infection (bubonic plague and human being).

28. Host: Types
• Link host is one which bridges the gap
between maintenance host and man (goats
between small mammals and man in case of
tick-borne encephalitis).

29. Environment
• Physical: Air, Temperature, Water etc
• Biological: Living beings
• Social: Cultural practices, Social controls

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Heredity
Lifestyles
Environment
Health
Services
Socioeconomic
Conditions

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Heredity
 Heredity & biological factors
 Abnormalities in chromosome
structure
 Abnormalities in chromosome
number
 Genetic inheritance

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Life Styles
Describes social milieu
(Psycho-social-economic
environment)
Repetitive patterns
of behavior

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Lifestyles
Include:
 Cultural patterns Positive
 Behavioural patterns Neutral
 Habits Negative

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Types of Life Styles
• Positive: Health promoting
To be encouraged
• Neutral: Effect on health uncertain
To be left as they are
• Negative: Deterrent to health
To be discouraged, removed

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Cultural Patterns
 Deep rooted
 Acceptance of community
 Some are subject to social controls
Examples
Positive: Bathing of child in morning sun
Negative: Application of cow dung to cord
Neutral: Applying flour on child’s skin before
bath

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Behavioural Patterns
• Effect of socialization
• Repetitive in nature
• Give predictability
Examples
Sexually promiscuous behaviour
Aggressive behaviour in type A personality

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Habits
• Accustomed way of doing things
• Acquired through repetition
• Performed under similar conditions

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Habits: Principles
 Early inception: Most are incepted
early in childhood.
 Repetitive nature: Are formed due
to repeated, frequent activities.
 Induction time: Take time to form.
 Stimulus: Mostly require an
emotional stimulus.

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Habits: Mechanism
1. Imitation:- Imitation of others
On whom dependent (parents) ,
Who are admired (heroes, idols)
2. Social controls:- Formal and informal
social controls play a role in formation of
habits. Habit of women from orthodox
families of speaking looking down is a
classical example.

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Habits: Mechanism
3. Knowledge:- The knowledge that one
can have scabies if one uses other's
clothes leads to 'habit' f not wearing
other's clothes.

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Habits: Types
• Positive: Taking regular exercise
• Neutral: Touching own nose while
speaking
• Negative: Biting nails

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Habits & Health
 Exposure to hazards: Eg. Smoking,
alcohol
 Indicative of personality: Nail biting
Nervous personality
 Positive habits: health
consciousness
v

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Social
Network
Culture Family History
Education Occupation Income
Personal Characteristics
Life Styles
Patterns of Behaviour Living
Condition
Health Care Behaviour
Disease RFs
Personal Characteristics Age,Sex, Race
Health
Services
Health Status

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Environment
Types:
 Physical: Air, Water, Noise, Radiation
 Biological: Living organisms like microbes ,
insects, rodents, animals
 Social: Persons around & their
interpersonal interactions

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Environment & Health
• Direct Effect: Eg. Effect of heat & humidity
• Indirect Effect:
 Harboring infectious agents
 Harboring chemical agents
 Creating conditions suitable for
infectious agent

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Socio-economic Conditions
Determined by various indicators like:
 Income
 Education
 Occupation
 Caste, Race
Methods of Socioeconomic Classification
Kuppuswami’s, Prasad’s, Parik’s, R.G’s

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S.E. Status & Health
• Direct effect: Nutritional disorders
• Indirect Effect
 Education Poverty Ignorance
 Poverty Poor environment &
hygiene
 Poverty Poor accessibility &
utilization of health services
v
v
v

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Health Services
Include:
 Personal & Community Services
 Promotive, Curative, Preventive &
Rehabilitative Services

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Health Services
Aspects that affect health
o Availability: Even at most peripheral
parts and to those who cannot afford
o Accessibility: Physical & Cultural
o Quality: Technically sound
o Utilization

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Other Factors
• Demographic:
Number, characteristics & distribution
Affect health needs
Decides population at risk
• Information & Communication:
• Systems outside health : Food &
Agriculture, Education, Industry, Social
Welfare