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Epidemiology is a discipline to study distribution (or occurrence and development) of diseases and health conditions in human populations and the determines influencing the distribution as well as to study the measures for the prevention and control of disease and promotion of health.
Cause of disease is the factors which can increase the incidence rate of disease. And if one or more of these factors don’t exist, the incidence rate of population will decrease.
In epidemiology, we can also describe these factors as risk factors. That means these factors can be indirect causes of disease .
(1)Organism Human Disease
only suitable for infectious diseases.
For example, tubercle germ---tuberculosis（TB）
However, the organism alone is an essential factor, but not a sufficient factor for infectious diseases.
For example, influenza virus influenza.
In fact, this model is also suitable for infectious diseases. But, in this model, environments are attended.
tubercle + low level of immunity
Social Environment diseases. But, in this model, environments are attended.
(3)The Wheel Model
The relative size of the different components of the wheel depend upon the specific disease.
For hereditary diseases, the genetic core would be relatively large.
For a disease likemeasles, the genetic core would be smaller, the state of immunity of the host and biological sector of environment would be larger.
Host Factor (Intrinsic)
(1) depend upon the specific disease. Genetic Factor
Most diseases have relationship with genetic factor , but the degree is different.
Some diseases only due to genetic factors, such as color blinding.
Some diseases depend upon the specific disease. partly due to genetic factors,partly due to environment factors.
For example, ABO blood type
Type A gastric cancer
Type O duodenal ulcer
(2) depend upon the specific disease. Immunity
Type A: CHD (coronary heart disease)
Type B: Cancer
Environmental Factors (Extrinsic depend upon the specific disease. )
such as bacteria, virus
In Japan stomach cancer
(3) depend upon the specific disease. Physical Environment
Including heat, light, air, water, radiation, atmospheric pressure, and so on.
Air pollution chronic respiratory disease, such as chronic bronchitis and lung cancer
Such as heavy metal, trace element.
Section 3 Process of Study on depend upon the specific disease.
Cause of Disease
1 depend upon the specific disease. Descriptive study to understand the distribution of disease formulate etiologic hypotheses
For example: smoking and lung cancer
For example: depend upon the specific disease. lung cancer
At the begin,we don’t know the cause of lung cancer.
Through descriptive study, we can know the characters of distribution of this disease.
Population: incidence rate
Place: depend upon the specific disease. consumption of tobacco is more,incidence rate is higher.
Time: current year> before time
Raise clue or hypotheses of cause:
True or not? Analytic Study
2 depend upon the specific disease. Analysis study and experimental Epidemiology
test the hypotheses
Frequency of smoking in group 1 ＞ group 2
Patients with lung cancer
But depend upon the specific disease. , We can not estimate cause-effect relationship.
Why? The direction of this kind study is retrospective, cause effect.
If we want to know whether there is cause—effect relationship, we must do cohort study.
Follow up depend upon the specific disease.
Incidence rate of lung cancer in smoking group >
Because the direction of cohort study is prospective,
Cause-effect relationship exits
Experimental Epidemiology depend upon the specific disease.
Experimental group depend upon the specific disease.
Example of intervention test:
Lung cancer ----- smoking
Give up smoking
Comparing incidence rate
If group 1＜ group 2
3 depend upon the specific disease. Judgment of A Cause-effect Relationship
In this chapter, we mainly talk about judgment of cause-effect relationship.
But ,before this we must do a work firstly,that is:
Section 4 Evaluation of Presence of depend upon the specific disease.
A Valid Statistical Association
In epidemiology study ,we can find relationship between cause and disease, but it is just a statistical association. That means it’s may be the true effect of an exposure on the development of disease, and it also possible has other explanation:
① it’s maybe spurious association
② it’s maybe indirect association
1 Spurious Association cause and disease, but it is just a statistical association. That means it
In the course of epidemiology such as design, selecting subjects, dividing groups,collecting data and analysis data, if there is difference between this two groups, the result we obtained is error, we can also call the relationship as spurious association.
For example cause and disease, but it is just a statistical association. That means it: smoking --- lung cancer
In case-control study
Patient group: 80% 20%
Control group: 50% 50%
Result : smoking frequency
patient group > control group
A cause and disease, but it is just a statistical association. That means it
2 Indirect association
smoking cause and disease, but it is just a statistical association. That means it
Section 5 Criteria in the Judgment of Cause of Disease cause and disease, but it is just a statistical association. That means it
There are about 6 criteria in the judgment of cause of disease. After epidemiological study has been done we shall use these criteria to deduce the cause of disease.
1 cause and disease, but it is just a statistical association. That means itTime Sequence
The exposure occurs in former and the disease occurs in later, this is an essential condition to judge a cause-effect relationship.
It is easy to judge time sequence in prospective cohort study , or randomized trail.
But in case-control study or cross-sectional study, it is difficult.
2 Strength of the Association study , or randomized trail.
We usually express strength of the association by use of risk ratio (RR) .
RR is more, the possibility of association between exposure and disease is larger.
incidence rate of exposure group
incidence rate of non exposure group
RR of smoking and lung cancer is 9 study , or randomized trail.
RR of smoking and CHD is only 2
It is showed that smoking is more likely the cause of lung cancer than CHD.
In general, if the design and analysis are all correct and RR is more than 3 or 4, we can believe that the exposure is the cause of this disease.
3 Biologic Credibility study , or randomized trail.
If there is a known or postulated mechanism which supports a cause and effect relationship, it can enhance the relationship.
For example, study , or randomized trail.smoking ---lung cancer
There were so many chemical carcinogens in tar, such as arsenic, carbon monoxide, and so on. Mean while , let dog inhale the cigarette smoking ,the dog occurred lung cancer.
4 study , or randomized trail.Reeligible of Association
To a cause-effect relationship, if different researchers use different methods in different time or different place among different populations, they all show similar or same results, this can support association much more.
For example study , or randomized trail.,
Doll and Hill smoking and lung cancer
more than 7 times cohort study
more than 35 times case-control study
5 study , or randomized trail.Dose-Response Relationship
When the degree of exposure has changed, if the morbidity of disease changed corresponding, the possibility of a cause-effect relationship is more large.
For example, the more amount population smoke every day, the higher mortality rate they dead from lung cancer.
6 If eliminating the exposure,the morbidity rate of disease will decrease.
For example, after having got rid of smoking ,we found incidence rate and mortality rate of lung cancer decrease.This enhance the persuasiveness to cause-effect association.
In fact, this list also corresponding the second sentence in definition of cause of disease.
These 6 are criteria when we judge a cause-effect relationship . But that is not to say, judging a relationship must accord with all of this 6 criteria.
Time sequence is essential. For the other 5criteria, the more criteria it accords with, the higher possibility the hypotheses establishes.