The document discusses the natural history of diseases, defining stages from pre-pathogenesis to pathogenesis, and emphasizes the importance of understanding these stages for effective disease prevention and screening. It outlines three levels of prevention: primary, secondary, and tertiary, detailing respective strategies for intervention at different disease stages. Additionally, it addresses concepts like lead time bias and length time bias in screening, and the necessity of prognostic evaluations in assessing disease treatment outcomes.

1. Natural History Of
Diseases and Levels of
Prevention
By : Sourav Goswami
Moderator : Dr P R Deshmukh
MGIMS, Sevagram

2. Framework
•1.Definition
•2.Understanding Natural History of
Disease
•3.Its Importance
•4.Stages
•5.Application : Levels of
Prevention/Screening/prognosis/Ev
aluation

3. Definition
•Natural History of a disease
signifies the way in which a disease
evolves over time from the earliest
stage of its prepathogenesis phase
to its termination as
recovery,disability or death,in the
absence of treatment or prevention.

4. Natural History of Typhoid
Entry of
S.typhi
Entry of
S.typhi
Incuba-
tion
period
14 days
Incuba-
tion
period
14 days
Palpabl
-e
spleen
,Rash
Palpabl
-e
spleen
,Rash
Headach
e, Fever,
Pea-
soup
stool
Headach
e, Fever,
Pea-
soup
stool
COMPLICATIONS:
Hemorrhage
Perforation
Peritonities
COMPLICATIONS:
Hemorrhage
Perforation
Peritonities
DEATH/
DISABIL-
ITY(carri
e-r)
DEATH/
DISABIL-
ITY(carri
e-r)
RecoveryRecovery

5. Natural history of disease

6. Natural history of Hepatitis B infection
SUSCEPTIBLE
HOST
SUSCEPTIBLE
HOST
ON
EXPOSURE
ON
EXPOSURE
ENTRY
OF HBV
ENTRY
OF HBV
DEVEL
-OP
HEP-B
DEVEL
-OP
HEP-B
OUTCOMEOUTCOME
C
I
R
R
H
O
S
I
S
C
I
R
R
H
O
S
I
S
HCCHCC
CARRIERCARRIER
DEATHDEATH

7. Susceptible
host
TIME
Incubation period
Death
Recovery
Exposure Onset
Latent Infectious Non-infectious
Infection
No infection
Clinical disease

8. • Incubation period
the time interval between invasion by an infectious
agent and appearance of the first sign or symptom
of the disease in question
Latent period
It is used in non-infectious diseases as the equivalent
of incubation period in infectious disease
-”Period from disease initiation to disease detection”
Infectious period
the time during which the host can infect another
susceptible host
• Non-infectious period
the period when the host’s ability to transmit
disease to other hosts ceases

9. Stages of Natural History
of Disease
The natural history of disease can
be divided into two stages :
1. Pre-pathogenesis phase
2. Pathogenesis phase

10. 1. Pre-Pathogenesis Phase / Stage
of susceptibility
In this stage, the disease has not developed
but the ground has been laid by the presence
of factors that favor its occurrence, for eg :
1.Alcohol consumption for Cirrhosis of liver
2.High Cholesterol, obesity, Type A personality:
Heart Disease
3.Smoking, Hypertension, High Cholesterol :
Stroke
4.Radiation, Smoking, Immune suppression:
Cancer

11. Pathogenesis phase
• 1. Asymptomatic (Early Pathogenesis)
phase
• 2. Early, Discernible Disease
• 3. Full-Blown (Classical) Disease
• 4. Termination - a) Complete Recovery
• b) Chronic Disease
• c) Life With Residual
Disability
• d) Death

12. Why is it important to study
natural history of disease?
•1. For planning preventive activities
•2.Adjusting lead time & length bias
for proper implementation of
screening program
•3.Forecasting prognosis
•4. Evaluation of intervention

13. Prevention

14. Levels of prevention
•In general, there are mainly three
major levels of prevention, depending
on the phase of the natural history of
the disease :
•1. Primary prevention (also primordial
prevention )
•2. Secondary prevention
•3. Tertiary prevention

16. • Primary prevention seeks to prevent the
onset of specific diseases via risk reduction:
• (a) by altering behaviors /exposures that can
lead to disease,(eg : cessation of smoking ) or
• (b) by enhancing resistance to the effects of
exposure to a disease agent (eg : Vaccination )
• It can be done by : (1) Health Promotion
(2) Specific protection

17. Health Promotion
• “The process of enabling people to
increase control over, and to improve
health” (WHO)
It is not directed against any particular disease,
but is intended to strengthen the host through
a variety of approaches :
• 1.Health education
• 2.Environmental modifications
• 3.Nutritional intervention
• 4.Lifestyle and behavioural changes

18. 1) HEALTH EDUCATION : Most cost effective
intervention. Now people have moved to behavior
change communication.
2) ENVIRONMENTAL MODIFICATION :
# provision of safe water
#Installation of sanitary latrines
#Control of insects & rodents etc
3)NUTRITIONAL INTERVENTIONS :
# Food distribution & nutritional
improvements of vulnarable groups ( viz. Mid-day
meal in schools ,Khichri on Anganwadi etc ) etc
4) LIFE STYLE & BEHAVIOURAL CHANGE :
# motivation for healthy lifestyle
Contd……

19. Specific protection
• immunization to protect against
specific diseases
• fortification of foods with specific
nutrients (as salt with iodine),
• use of condoms to protect
against STDs,
• use of chemoprophylactic drugs
to protect against particular
diseases (as malaria,
meningococcal meningitis, etc)

20. #Primordial Prevention
• This is prevention of the emergence of risk
factors in countries or population groups in
which they haven't yet appeared.
• By “individual and mass education”
• It addresses BROAD HEALTH
DETERMINANTS rather than preventing
personal exposure to risk factors, which is
the goal of primary prevention.

21. Contd ……
•Thus, outlawing alcohol in
certain countries/areas would
represent primordial
prevention, whereas
•a campaign against drinking and
would be an example of
primary prevention.

22. Secondary prevention
• It include all actions undertaken at the
stage of early pathogenesis so as to halt
the progress of disease at it’s earliest
stage,
• It is done by “early diagnosis and
prompt treatment”
• eg : Screening for Cancer/ treatment of
Tuberculosis-early diagnosis & prompt
treatment/Diagnosis & treatment of
malaria

23. Tertiary Prevention
• It signifies interventions done in the late
pathogenesis phase.
• “All measures available to reduce or limit
impairments and disabilities,minimise
sufferings caused by existing departures from
good health and to promote the patient’s
adjustment to irremediable conditions”
( Last,, A Dictionary of Epidemiology )
• It can be attained by : a) Disability limitation
& b) Rehabilitation

24. Disability limitation
(impairment/disability/handicap
)
•Impairment is defined as "any loss or
abnormality of psychological,
physiological, or anatomical
structure or function."
• Impairment is a deviation from
normal organ function; it may be
visible or invisible (screening tests
generally seek to identify
impairments).

25. • Disability is defined as "any restriction or lack
(resulting from an impairment) of ability to
perform an activity in the manner or within
the range considered normal for a human
being."
• An impairment does not necessarily lead to a
disability, for the impairment may be
corrected.
• For example, I am wearing eye glasses, but do
not perceive that any disability arises from my
impaired vision. A disability refers to the
function of the individual (rather than of an
organ, as with impairment).

26. • Handicap is defined as "a disadvantage for
a given individual, resulting from an
impairment or a disability, that limits or
prevents the fulfillment of a role that is
normal (depending on age, sex, and social
and cultural factors) for that individual."
• Handicap considers the person's participation
in their social context.
• For example, if there is a wheel-chair
access ramp at work, a disabled person
may not be handicapped in coming to work
there

27. Disability limitation
• Concept of disability:
DISEASE
Accident
(1)
DISEASE
Accident
(1)
IMPAIRMENT
Loss of foot
(2)
IMPAIRMENT
Loss of foot
(2)
DISABIL-ITY
Cannot walk
(3)
DISABIL-ITY
Cannot walk
(3)
HANDICAP
Unemploye
d
(4)
HANDICAP
Unemploye
d
(4)

28. Contd…
• Disability limitation includes all measures
to prevent the occurrence of further
complications, impairments, disabilities
and handicaps or even death. For
example :
• When we apply plaster cast to a patient
who has suffered Colle’s fracture, we are
actually trying to prevent complications
and further disability like mal-union or
non-union (4)

29. Rehabilitation
• “Rehabilitation” (Re =restore into,
habitat = the original home or
environment of the person)
• “The combined and coordinated use of
medical,social,educational and vocational
measures for training and retraining the
individual to the highest possible level of
functional ability”
• It includes Physiotherapy,speech
therapy,audiology,psychology, vocational
work etc

30. Rehabilitation contd…
• The following areas of concern have been
identified :
• 1)Medical rehabilitation – restoration of
function
• 2)Vocational rehabilitation- restoration
of the capacity to earn a livelihood
• 3)Social rehabilitation –restoration of
family & social relationships
• 4)Psychological rehabilitation –
restoration of personal dignity and
confidence

31. Examples of rehabilitation
•Establishing schools for the blind,
•provision of aids for the crippled,
•reconstructive surgery in leprosy,
•change of profession for a more
suitable one etc

32. Knowledge of Natural History
of disease helps in adjusting
lead-time & length
bias…..which helps in
implementing proper
screening measures

33. Lead time bias in a screening
study….........

34. Course of a disease &
how screening works…
Pathology
Begins
Symptom
appears
DEATH
/
DISABI
LITY
DEATH
/
DISABI
LITY
SCREENING TEST &
EARLY DIAGNOSIS
(pre-symptom)
X

35. Lead time bias
• Lead Time :
Time between detection by screening and
symptoms development/diagnosis
If we start counting years of survival
from date of diagnosis , moving the
date of diagnosis earlier , makes
survival appear longer even if
treatment is ineffective.

37. •Solving lead time bias problem :
•Compare age specific mortality
between screened and not-screened.
•Not survival!
•Count from the date of
randomisation.

38. Length time bias…..

39. Length time bias
•Screening picks up prevalent disease
- Prevalence = Incidence X Duration
Slowly growing tumors have greater
duration in presymptomatic phase,
therefore greater prevalence

40. Length-time Bias
Aggressive Disease
Onset Clinical
Presentation
Death
Clinical
Presentation
DeathOnset
1 yr sympto
Screening interval
1 year
6 mo.asymt
period
2 year asym
period
4 yr sym
Less Aggressive Disease

41. • Therefore, cases picked up by screening
,will be disproptionately those, that are
slower growing.
• Slower growing tumors tend to have
longer survival (better prognosis)
independent of treatment .
• Overestimation of survival duration among
screening detected cases caused by the
relative excess of slowly progressing cases.

43. AVOIDING LENGTH TIME BIAS
•Compare outcome via RCT with a
control group and a group
offering screening
•Count all outcomes regardless of
method of detection

44. Prognosis: How much time do I
have doc???

45. •Prognosis is the
prediction of the course of
a disease
and
•is expressed as the
probability that a
particular event will occur
in the future

46. Prognosis contd …..
• Predictions are based on defined groups of
patients and the outcome may be quite
different for the individual patients
• However, knowledge of the likely prognosis
is helpful in determining the most useful
treatment.
• Prognostic factors are characteristics
associated with outcome in patients with
the disease in question.
• For example, for a patient with AMI, the
prognosis is directly related to heart muscle
function.

47. Rates commonly used to describe Prognosis
Rate What does it mean ?
5 yr survival Percentage of patients who are alive 5 years
after treatment begins or 5 years after
diagnosis
Case fatality Percent of patients with a disease who die of it
Disease-sp.
mortality
Number of people per 10,000 (or 100,000)
population dying of specific disease
Response Percent of patients showing some evidence of
improvement following an intervention
Remission Percent of patients entering a phase in which
disease is no longer detectable
Recurrence Percent of patients who have return of disease
after a disease free interval

48. Application of natural history of
disease : Evaluation of interventional
measures
Evaluation helps in
1)Providing feedback on the
effectiveness of a intervention
2)helps to determine whether the
program is appropriate for the target
population

49. • 3) is there any problems with its
implementation and support, and
• 4)whether there are any ongoing
concerns that need to be resolved as
the programme is implemented.
•5)It helps in Comparing intervention
modalities

50. Reference
• 1) AFMC (Association of Faculties of
Medicine of Canada) Primer on Population
Health-A virtual textbook on Public Health
concepts for clinicians
• 2)Epidemiology by Leon Gordis( Fifth
Edition)
• 3)Park’s testbook of Preventive and social
Medicine( 23rd
edition )
• 4)Text book of Public Health and community
medicine by Armed Force Medical College

51. Thank you !