1. Distribution of Disease
EPIDEMIOLOGY
What is Epidemiology?
Distribution of Disease
WHEN Time
WHERE Place
WHO Person

2. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
Time
Changing or stable
Cyclic
Seasonal
secular

3. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
Place
Geographic
-Restricted/Wide spread
-Coastal/inland
Weather(temp. , humidity , rainfall)
Altitude/latitude/longitude
Urban/rural

4. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
Person
Age
Sex
Race
Education
Occupation
behavior

5. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
morbidity rates
Incidence rate
The numerator is new cases
The denominator may change over time as people develop disease
However, in practice
in large studies, the denominator is often the mid- year population
in small studies, the denominator includes persons at risk of the disease

6. Number of episodes during
EPIDEMIOLOGY
Distribution of Disease
morbidity rates
Attack rate عليه مسائل
Number of episodes during
specified period x
10n
Attack rate =
Population at risk/
period
Acute recurrent diseases ,
e.g. URTI, food poisoning.
A person may catch the disease more than once.

7. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
morbidity rates
Attack rate
Example:
In a village of 600 persons, visits to the PHCC were due to food poisoning.
What is the attack rate of food poisoning
in the village ?

8. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
morbidity rates
Attack rate
Example:
In a village of 600 persons, visits to the PHCC were due to flu.
What is the attack rate of flu
in the village ?

9. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
morbidity rates
Incidence rate
Factors affecting incidence:
1- New risk factor
Contraceptive pills & thromboembolism.
2- Changing virulence
H1N1virus & influenza pandemic
3- Changing pattern of intervention
Chemoprophylaxis & meningitis

10. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
morbidity rates
Incidence rate
Factors affecting incidence:
4-Population pattern
Aging & degenerative diseases & cancer
5- Reporting
incidence
Increase in reporting &
Early stage of surveillance &
incidence

11. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
morbidity rates
Incidence rate
Factors affecting incidence:
6-Screening
7-New diagnostics
Screening
New diagnostic
early detection
of cases
detection of cases
incidence
incidence

12. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
morbidity rates
Incidence rate
Factors affecting incidence:
8- Changing classification
Blood pressure
140/80 mmhg Vs /90 mmhg
incidence
Fasting blood sugar
126 mg/100ml Vs mg/100ml

13. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
morbidity rates
Prevalence rate مسائل وتفهمي ايش يعني
Point prevalence
Period prevalence
It involves old and new cases
It measures the disease status in the population.
It measures the disease burden.
( All Cases)

14. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
morbidity rates
Prevalence rate
Point
prevalence:
Total cases (old + new)
at fixed point of time
in a place = X
1On
total population
in that place

15. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
morbidity rates
Prevalence rate
Period
prevalence:
Total cases (old + new)
during a period of time
in a place = X
1On
total population
in that place

16. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
morbidity rates
prevalence rate
example
MOH conducted a survey for RVF among workers in slaughterhouses in Makkah seropositive workers were identified among 6000 workers.
Calculate the prevalence of RVF.

17. Distribution of Disease
EPIDEMIOLOGY
Distribution of Disease
morbidity rates
prevalence rate
Factors affecting prevalence :
1- Incidence rate
& 2- Disease duration
average
duration of
disease
Prevalence
rate
Incidence
rate = X
Incidence
Prevalence
Longer duration
Prevalence