1. Natural History of Disease

2. Background Infectious disease epidemiology
the occurrence of infectious disease in a given host is dependent on the presence of disease in other members of the population and the length of time that infected hosts are able to transmit disease to others
understanding these characteristics of a
disease allow us to develop rational measures to control disease

3. Definition & Stages
Definition ; The course of a disease from onset (inception) to resolution.
Stages
Stage of
pathologic
onset
Pre-symptomatic
stage
Clinically
manifest disease
Progress to a fatal termination
Remission and relapses
Regress spontaneously,
leading to recovery
Risk Factors
Precursors
Effect of Treatment
Prognostic factor

4. Risk factor
Risk factor; An aspect of personal behavior or life style, an environmental exposure, or an inborn or inherited characteristic, that, in the basis of epidemiologic evidence, is known to be associated with health-related condition (s) considered important to prevent.
Risk marker; increased probability of a specified outcome; not necessarily a causal factor
Determinant; can be modified by intervention, thereby reducing the probability of occurrence of disease or other specified outcomes

5. The Natural history of disease in a patient
Preclinical Phase
Clinical Phase
(A)
(P)
(S)
(M)
(D)
(T)
A ; Biologic onset of disease
P ; Pathologic evidence of disease if Sought
S ; Signs and symptoms of disease
M ; Medical care sought
D ; Diagnosis
T ; Treatment
Gordis L. Epidemiology. WB Saunders Company. 1996

6. THE NATURAL HISTORY OF A DISEASE STIMULUS to the HOST HOST REACTION
RECOVERY
interrelation of Agent, Host and Environmental factors
Latent Period (Pre-symptomatic)
Symptoms, Signs(Clinical)
with or without Defects, Disability
PREPATHOGENESIS
PERIOD OF PATHOGENESIS
Health Promotion
Specific Protection
Early Diagnosis and Prompt Treatment,
Disability Limitation
Rehabilitation
PRIMARY PREVENTION
SECONDARY PREVENTION
TREATMENT
TERTIARY PREVENTION
(Leavell's Level of Application of Preventive Medicine)

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

8. Latent period
the time interval from infection to development of infectiousness
Infectious period
the time during which time the host can infect another susceptible host
Non-infectious period
the period when the host’s ability to transmit disease to other hosts ceases
Incubation period
the time interval between infection to development of clinical disease

9. e.g : Chicken pox
an infectious disease caused by the varicella-zoster virus
the latent period for chicken pox is shorter than the incubation period, so a child with chicken pox becomes infectious to others before developing symptoms

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

11. Other examples? HIV (AIDS) latent period relatively short
infectious period occurs (many years) before the onset of symptoms

12. TIME Onset Exposure Incubation period Latent Infectious
Death
Recovery
Clinical disease
Infection
No infection
Susceptible
host
Onset
Exposure
Incubation period
Latent
Infectious

13. e.g : Malaria caused by protozoan parasites of the genus Plasmodium
the stages of the parasite that are infective to mosquitoes occur about 10 days after the development of symptoms
latent period is around 10 days longer than the incubation period, so early treatment of symptoms could have an important effect on transmission

14. Natural history of disease
TIME
Death
Recovery
Clinical disease
Infection
No infection
Susceptible
host
Onset
Exposure
Incubation period
Latent
Infectious

15. Latent Period of Chronic Disease
Definition; "Interval between exposure to a disease-causing agent and the appearance of manifestations of the disease"
cf. incubation period in infectious disease
1) brief exposure
Two conditions
2) prolonged or continuous exposure

16. Primary Prevention
'Preventing the occurrence of disease or injury by modifying risk factors.'
'Various aspects are considered to produce effective primary prevention program. Especially, advancing knowledge of disease causation must be required.‘

17. Primary Prevention
** Guidelines for effective prevention programs(RB Wallace, GD Everett,1986)
Programs must be based on scientific evidence.
Prevention programs should be supported by effective data system.
Programs should be flexible.
Programs must be sensitive to ethical issues.
Programs should be targeted to the recipients most in need.
Programs should muster a variety of community resources.
Effective prevention requires legislative action and social policy decisions.
Programs should be continuous.

18. Primary Prevention General health promotion Specific protection
'Proper nutrition, mental hygiene, adequate housing, and appropriate balance between work and play, est and exercise, and useful and productive place in society, are among the best recognized factors ontributing to maintenance of optimum health.(Commission on Chronic illness, USA, 1957)‘
Specific protection
Health Promotion
'Health promotion is any combination of educational, organizational, economic, and environmental supports for behavior and conditions of living conducive to health (LW Green, 1992).'

19. Criteria for the Development of Health Promotion and Education Programs
A health promotion program should address one or more risk factors which are carefully defined, measurable, modifiable, and prevalent among the members of a chosen group, factors which constitute a threat to the health status and the quality of life of target group members.
A health promotion program should reflect a consideration of the special characteristics, needs, and preferences of its target groups(s)
From APHA Technical Report

20. Criteria for the Development of Health Promotion and Education Programs
health promotion programs should include interventions which will clearly and effectively reduce a targeted risk factor and are appropriate for a particular setting
A health promotion program should identify and implement interventions which make optimum use of available resources.
From the outset, a health promotion program should be organized, planned, and implemented in such a way that its operation and effects can be evaluated.

21. Secondary Prevention
'Early detection and intervention, preferably before the condition is clinically apparent, and has the aim of reversing, halting, or at least retarding the progress of a condition.‘
'It sometimes happens that a patient first becomes aware of a disease when it is already too late for it to be successfully treated.‘

22. Tertiary Prevention
'Minimizing the effects of disease and disability by surveillance and maintenance aimed at preventing complications and premature deterioration'
Medical rehabilitation
Social rehabilitation

23. THANK YOU