1. Chain of Infection

13. Disease causation

14. Cause of disease:
is "an event, condition, characteristic or a
combination of these factors, which plays an
important role in producing the disease".

15. The causes of disease can be classified into two:
1. Primary causes: factors that are necessary for a disease to occur,
in whose absence the disease will not occur. The term "etiologic
agent" can be used instead of primary cause for Infectious causes
of diseases. For example, “Mycobacterium tuberculosis” is the
primary cause (etiologic agent) of pulmonary tuberculosis.
2. Risk factors: factors that are not the necessary causes of disease
but they are important for a disease to occur. Risk factors could be
related to the agent, the host and the environment.
so, the etiology of a disease is the sum total of all the factors
(primary causes and risk factors) which contribute to the occurrence
of the disease.

16. Models of disease causation:
1. Supernatural theory of disease:
• In the early past, the disease was thought either mainly due to the curse of god or
due to the evil force of the demons. Accordingly, people used to please the gods by
prayers and offerings or used to resort to witchcraft to tame the devils.
2. Ecological theory:
• Hippocrates is the first epidemiologist who advised to search the environment for
the cause of the disease.
• Environmental influence: interactions among humans, other living creatures, plants,
animals, microorganisms, ecosystems, geography and climate.

17. 3. Germ theory:
• Microbes (germs) were found to be the cause for many known diseases.
Pasteur, Henle and Koch were the strong proponents of microbial theory
after they discovered the microorganisms in the patients’ secretions or
excretions.
Henle-Koch postulates; sometimes called “pure determinism”
1. The agent is present in every case of the disease
2. Without that agent, that disease will not be caused (one agent - one disease).
3. It can be isolated.
4. If exposed to healthy subjects will cause the related disease
Causative agent Host Disease

18. 4. Epidemiologic Triad:
According to this theory, not only the causative agent is
responsible of causing the disease but also there are
other factors related to the host and environment.
Environment
Host
Agent
• Agent of infection
• Host
• Environment
Disease
AGENT
HOST
ENVIRONMENT

19. 5.Epidemiologic Tetrad:
Time is very important factor in disease
causation.
• Agent of infection
• Host
• Environment
• Time

20. 6. BEINGS (or BBEINGSSS) theory:
This theory postulates that human disease and its
consequences are caused by a complex interplay of
(9) different factors:
• Biological factors
• Behavioral Factors
• Environmental factors
• Immunologic factors
• Nutritional factors
• Genetic factors
• Services and Social factors,
• and Spiritual factors

21. 7. Multi-factorial causation theory:
This theory helps to address various associated causative
factors of a disease.
8. Epidemiological web causation:
McMahon and Pugh used this model to illustrate various
factors contributing to a disease wherein these factors are
linked like an interacting web of spider.
 Each factor has its own relative importance in causing the
final deviation from the state of health, as well as interact
with each other and modifying the effect of each other.

22. Web of Causation for the Major Cardiovascular Diseases

23. Web of Causation for pulmonary tuberculosis

24. 9. Wheel causation:
• This theory has been postulated to explain the difference between
agent (intrinsic) factors and environmental (extrinsic) factors in
causing a disease.
• The theory visualize a human disease as a wheel, which has a central
hub representing the genetic components and peripheral portions
representing environmental components.

25. Types of Causal Relationships:
1. Necessary and
sufficient
2. Necessary but not
sufficient
3. Sufficient but not
necessary
4. Neither sufficient
nor necessary
• Without the
factor, disease
never develops
• With the factor,
disease always
develops (this
situation rarely
occurs)
• The factor should
be present in, but
of itself is not
enough to cause
disease
• Multiple factors
are required,
usually in a specific
temporal sequence
(such as
carcinogenesis)
• The factor alone
can cause disease,
but so can other
factors in its
absence
• Benzene or
radiation can cause
leukemia without
the presence of
the other
• The factor cannot
cause disease on
its own, nor is it
the only factor that
can cause that
disease
• This is the
probable model for
chronic disease
relationships

26. Factors in Causation
Four types of factor play a part in causation of disease. All may be necessary but
will rarely be sufficient to cause a particular disease.
1. Predisposing factors– create a state of susceptibility, so the host become
vulnerable to a disease agent or necessary cause. e.g. age, sex or previous
illness.
2. Enabling factors – conditions that favor the development of disease. e.g. low
income, poor nutrition, bad housing or inadequate medical care.
• Conversely, circumstances that assist in recovery or in health maintenance
may be enabling factors.

27. 3. Precipitating factors – those associated with
immediate exposure to a disease or noxious agent
or onset of disease. e.g. drinking contaminated
water, inhaling pollens in asthmatic attack or close
contact with a TB patient.
4. Reinforcing factors – factors that aggravate an
already established disease or state. e.g. repeated
exposure, malnutrition, undue work or stress.

28. Thank you!