This document discusses natural history and prognosis in epidemiology. It defines natural history as the stages of a disease, including the presymptomatic stage, clinical disease stage, and recovery/disability/death stage. Prognosis is defined as the prediction of a disease's course and the probability of future events, though outcomes may differ for individuals. Prognosis is based on patient groups and helps determine treatment. Factors like residual heart function affect prognosis for myocardial infarction patients. Prognosis assessments should include quality of life measures in addition to mortality. Randomly selected patient groups are needed to determine natural history and prognosis to avoid selection bias.

1. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD
Third year
Level 6
Lecture 4
Natural history and prognosis

2. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD
• Identify the natural history of disease.

3. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD
The term natural history refers to the stages of a
disease, which include:
1- Presymptomatic stage:
–Presence of pathogenic changes (biological onset)
–No disease manifestations
2- Stage of clinical disease
– The disease is clinically obvious
3- Stage of recovery, disability, or death

4. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD
• 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.
• Predictions are based on defined groups of patients, and
the outcome may be quite different for individual patients.
• However, knowledge of the likely prognosis is helpful in
determining the most useful treatment.
–For example, in a patient with acute myocardial infarction, the
prognosis is directly related to residual heart muscle function.

5. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD
• Ideally, the assessment of prognosis should include
measurement of all clinically relevant outcomes and not just
death, since patients are usually as interested in the quality of
life as they are in its duration.
– Stroke may have low mortality rate, but it affects the quality of life
seriously.
• In studies to determine natural history and prognosis, the group
of patients should be randomly selected; otherwise selection
bias may compromise the quality of information obtained.
– For example, the prognosis of patients with chest pain admitted to
hospital is likely to be worse than that of patients with chest pain seen
by health workers in the community.

6. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD

7. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD

8. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD

9. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD
• Prognosis in terms of mortality is measured as case-
fatality rate or probability of survival.
• Prognosis is measured by survival analysis

10. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD
This figure shows
significantly more people in
the cohort (1991–92)
survived three years after
a myocardial infarction than
did their counterparts 10
years earlier, which suggests
improvement in secondary
prevention of coronary
heart disease.

11. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD

12. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD
• A risk factor refers to an aspect of personal habits or an
environmental exposure, that is associated with an increased
probability of occurrence of a disease.
• Since risk factors can usually be modified, intervening to alter
them in a favourable direction can reduce the probability of
occurrence of disease.
• Risk factors include unsafe sex, tobacco and alcohol use, diet,
physical inactivity, blood pressure and obesity.
• Since risk factors can be used to predict future disease, their
measurement at a population level is important, but also
challenging.

13. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD
• Tobacco use can be measured by self-reported exposure (yes/no),
quantity of cigarettes smoked, or by biological markers (serum
cotinine).
• However, different surveys use different methods, often with
different measurement techniques and criteria for detecting a risk
factor or clinical outcome (for example, diabetes or hypertension).
• Additionally, surveys may only be representative of small
population groups within a country, district or city.
• These methodological differences mean that it is be difficult to
compare results from different surveys and countries.

14. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD
More recently, the WHO STEPS approach to the measurement of population levels of
risk factors provides methods and materials to encourage countries to collect data in a
standardized manner.

15. EPIDEMIOLOGY & COMMUNITY MEDICINE
WRITTEN AND COMPILED BY DR. EMAN ABD ALHALIM MD, PHD
Basic epidemiology
R Bonita, R Beaglehole
and T Kjellström
2nd edition 2006
World Health Organization
ISBN 92 4 154707 3
ISBN 978 92 4 154707 9
e-mail: permissions@who.int