Causation. A number of models of disease causation have been proposed. Among the simplest of these is the epidemiologic triad or triangle, the traditional model for infectious disease. The triad consists of an external agent, a susceptible host, and an environment that brings the host and agent together.

1. Causation of disease

2. CAUSATION OF DISEASE
What is cause?
Rothman :an event ,condition ,or characteristic with
out it the disease would not have occurred.
A cause of disease : can be defined as a factor
(characteristic ,behavior ,event… etc) that
influences the occurrence of disease.
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3. CHARACTERISTICS OF
CAUSE
 Must precede the effect
 A one to one correspondence between the observed cause and the
effects.
 Can be either host or environment factors
 Positive (presence of a causative exposure ,smoking for lung cancer) or
negative (lack of a preventive exposure ,immunization for measles)
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4. A CAUSE CAN BE
Necessary cause
Sufficient cause
Component cause
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5. NECESSARY CAUSE
It a component cause that is member of every
sufficient cause .
If you have A is necessary for B , it means that
every time you have B you will have A, with out
exception . A B
Examples
Tubercle bacillus is necessary factor for
tuberculosis.
With out oxygen ,we could not live ,there for
oxygen is necessary condition for human life.
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6. SUFFICIENT CAUSE
 a set of condition with out any one of which the disease would
not have occurred.
 If you have A is sufficient for B it means that every time A you
will have B ,with out exception . A B
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7. COMPONENT CAUSE
 Any one of the set of condition which are necessary for the
completion of a sufficient cause(this is piece of the pie)
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8. CAUSATION
A number of models of disease causation have been
proposed. Among the simplest of these is the
epidemiologic triad or triangle, the traditional
model for infectious disease. The triad consists of
an external agent, a susceptible host, and
an environment that brings the host and agent
together.
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9. CONT..
In this model, disease results from the interaction
between the agent and the susceptible host in an
environment that supports transmission of the agent
from a source to that host. Two ways of depicting
this model are shown in Figure 1.16.
It can help your students understand infectious
diseases and how they spread.
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10. EPIDEMIOLOGIC TRIAD
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11. CONT..
In addition, the accepted models of disease
causation all require the precise interaction of
factors and conditions before a disease will occur.
Finally, the concept of cause itself continues to be
discussed as a philosophical matter in the scientific
literature.
Nonetheless, the following models and guidelines
provide a framework for considering causation at a
practical level.
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12. CONT..
we will define a cause of disease as a factor
(characteristic, behavior, event, etc.) that influences
the occurrence of disease. An increase in the factor
leads to an increase in disease.
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13. CONT..
Reduction in the factor leads to a reduction in
disease. If disease does not develop without the
factor being present, then we term the causative
factor “necessary.” If the disease always results
from the factor, then we term the causative factor
“sufficient.”
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14. The Epidemiologic Triad: Agent,
Host, and Environment
The epidemiologic triangle or triad is the traditional
model of infectious disease causation. It has three
components:
 an external agent,
 a susceptible host, and
 an environment that brings the host and agent
together.
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15. CONT..
In this model, the environment influences the
agent, the host, and the route of transmission of the
agent from a source to the host.
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16. CONT…
Agent factors
Agent originally referred to an infectious
microorganism—virus, bacterium, parasite, or other
microbe. Generally, these agents must be present
for disease to occur. That is, they are necessary but
not always sufficient to cause disease.
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17. CONT..
As epidemiology has been applied to noninfectious
conditions, the concept of agent in this model has
been extended to include chemical and physical
causes of disease.
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18. CONT..
Host factors
Host factors are basic factors that influence an
individual’s exposure, susceptibility, or response to
a causative agent. Age, race, sex, socioeconomic
status, and behaviors (smoking, drug abuse,
lifestyle, sexual practices and contraception, eating
habits) are just some of the many host factors
which affect a person’s chance of exposure.
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19. CONT…
Age, genetic composition, nutritional and
immunologic status, anatomic structure, presence
of disease or medications, and psychological
makeup are some of the host factors which affect a
person’s susceptibility and response to an agent.
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20. CONT..
Environmental factors
Environmental factors are extrinsic factors which
affect the agent and the opportunity for exposure.
Generally, environmental factors include:
Physical factors such as geology, climate, and
physical surroundings (e.g. hospital);
Biologic factors such as insects that transmit the
agent ;and socioeconomic factors such as
crowding, sanitation, and the availability of
health services.
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21. CONT…
Agent, host, and environmental factors interrelate
in a variety of complex ways to produce disease in
humans. Their balance and interactions are different
for different diseases. When we search for causal
relationships, we must look at all three components
and analyze their interactions to find practical and
effective prevention and control measures.
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22. Component of Causes
Because the agent-host-environment model does
not work well for some noninfectious diseases,
several other models have been proposed. One of
the newer models is based on the multifactorial
nature of causation in many diseases.
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23. CONT..
A disease may have more than one sufficient cause,
with each sufficient cause being composed of
several factors. These include basic host factors, as
well as the agent and the environmental factors of
the agent-host- environment model.
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24. CONT…
Another example, lung cancer may result from a
sufficient cause which includes smoking as a
component cause. Smoking is not a sufficient cause
by itself, however, since not all smokers develop lung
cancer. Neither is smoking a necessary cause,
because lung cancer may occur in persons who never
smoked.
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25.  1. How do you define epidemiology?
 2. What are the purposes of epidemiology?
 3. What is natural history of diseases? What is the importance
of studying natural history of diseases?
 4. Briefly describe the cause of a disease?
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26.  Below are four key terms taken from the definition of epidemiology,
followed by a list of activities that an epidemiologist might perform.
Match the term to the activity that best describes it. You should
match only one term per activity.
A. Distribution
B. Determinants
C. Application
D. Frequency
------1. Compare food histories between persons with
Staphylococcus food poisoning and those without
----2. Compare frequency of brain cancer among anatomists with
frequency in general population
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27. Thank you!
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