1. Disease Causation
Bhoj R Singh
Section of Epidemiology, CADRAD, IVRI,
Izatnagar-243122, India

2. Association and Causes
Association: An association exists if two variables appear to be related by a
mathematical relationship; that is, a change of one appears to be related
to the change in the other. Association is necessary for a causal
relationship to exist but association alone does not prove that a causal
relationship exists. A correlation coefficient or the risk measures often
quantify associations.
– Negative Association (Inverse Relationship): The magnitude of one
variable appears to move in the opposite direction of the other associated
variable. The correlation coefficient is negative and, if the relationship is
causal, higher levels of the risk factor are protective against the outcome.
– Positive Association (Direct Relationship): The magnitudes of both
variables appear to move together up or down. The correlation coefficient
is positive and, if the relationship is causal, higher levels of the risk factor
cause more of the outcome.
•
Cause: The combination of necessary and sufficient factors (e.g., attributes
and exposures) the presence of which, alone or in combination, at some
time during an individual’s life, inevitably result in disease in that
individual.

3. CausesEtiology: The study of disease causes and their modes of operation.
Causal Pathway (Causal Web, Cause and Effect Relationships): The
actions of risk factors acting individually, in sequence, or together
that result in disease in an individual. These pathways are often
different with different sets of risk factors for individuals in
different situations. Understanding these pathways and their
differences is necessary to devise effective preventive or corrective
measures (interventions) for a specific situation. What is effective
in one pathway may not be in another because of the differences
in the component risk factors. (e.g., bronchopneumonia in a
housed calf vs. in a feedlot calf).
Necessary Cause: A risk factor that must be, or have been, present
for the disease to occur (e.g., a specific infectious agent for a
particular infectious disease). Although necessary, few infectious
agents cause disease by themselves alone.
Sufficient Cause: The minimal combination of risk factors acting on
the individual, on the etiologic agent if one is involved, or in the
environment whose occurrence in an individual’s life inevitably
results in disease. A disease can often be caused by more than one
set of sufficient causes and thus different causal pathways for
individuals contracting the disease in different situations.

4. Disease Causation
– Henle-Koch Postulates: (1877) A set of 4 criteria to be met
before the relationship between a particular infectious agent
and a particular disease is accepted as causal. These postulates
enabled the germ theory of disease to achieve dominance in
medicine over other theories, such as humors and miasma.
They are insufficient for multi-causal and non-infectious
diseases because the postulates presume that an infectious
agent is both necessary and sufficient cause for a disease.
Fulfilling the postulates experimentally can be surprisingly
difficult, even when the infectious process is thought to be well
understood. Now archaic and superseded by the Hill's-Evans
Postulates.
– Hill-Evans Postulates: (1965) A set of 9 or 10 criteria (depending
on interpretation of original papers) that each contribute a
different amount of strength to the likelihood that a relationship
between a potential risk factor and a disease is causal. The
entire set constitutes very strong evidence of causality when
fulfilled. As noted above, these supersede the Henle-Koch
Postulates and are extensions of Mill’s Methods of inductive
inference.

5. Inductive Inference for discovering
causal relationships
• Mill's Eliminative Methods of Induction (System of
Logic, 1843):
– Method of Agreement: "If two or more instances of the phenomenon
have only one circumstance in common, the circumstance in which alone
all instances agree is the cause or effect of the given phenomenon."
– Method of Difference: "If an instance in which the phenomenon under
investigation occurs, and an instance in which it does not occur, have
every circumstance in common save one, that one occurring in the
former, the circumstance in which alone the two instances differ, is the
effect, or the cause, or an indispensable part of the cause, of the
phenomenon."
– Method of Residues: "Subduct from any phenomenon such part as is
known by previous inductions to be the effect of certain antecedents,
and the residue of the phenomenon is the effect of the remaining
antecedents."
– Method of Concomitant Variations: "Whatever phenomenon varies in
any manner whenever another phenomenon varies in some particular
manner, is either a cause or an effect of that phenomenon, or is
connected with it through some fact of causation.“
– Method of Analogy: If it happens like that the similar other event should also
take the same route. Through comparison of patterns of the diseases.

6. Koch's postulates are
The postulates were formulated by Robert Koch and Friedrich Loeffler
in 1884 and refined and published by Koch in 1890.
• The microorganism must be found in abundance in all organisms
suffering from the disease, but should not be found in healthy
organisms.
• The microorganism must be isolated from a diseased organism and
grown in pure culture.
• The cultured microorganism should cause disease when introduced
into a healthy organism.
• The microorganism must be re-isolated from the inoculated,
diseased experimental host and identified as being identical to the
original specific causative agent.
• However, Koch abandoned the universalist requirement of the first
postulate altogether when he discovered asymptomatic carriers of
cholera and, later, of typhoid fever.

7. Koch's postulates for 21st Century:
The use of new methods in disease diagnosis has led to revised versions of Koch’s
postulates: Fredricks and Relman (1996) have suggested the following set of
Koch’s postulates for the 21st century:
• A nucleic acid sequence belonging to a putative pathogen should be present in
most cases of an infectious disease. Microbial nucleic acids should be found
preferentially in those organs or gross anatomic sites known to be diseased, and
not in those organs that lack pathology.
• Fewer, or no, copies of pathogen-associated nucleic acid sequences should occur
in hosts or tissues without disease.
• With resolution of disease, the copy number of pathogen-associated nucleic acid
sequences should decrease or become undetectable. With clinical relapse, the
opposite should occur.
• When sequence detection predates disease, or sequence copy number correlates
with severity of disease or pathology, the sequence-disease association is more
likely to be a causal relationship.
• The nature of the microorganism inferred from the available sequence should be
consistent with the known biological characteristics of that group of organisms.
• Tissue-sequence correlates should be sought at the cellular level: efforts should be
made to demonstrate specific in situ hybridization of microbial sequence to areas
of tissue pathology and to visible microorganisms or to areas where
microorganisms are presumed to be located.
• These sequence-based forms of evidence for microbial causation should be
reproducible.

8. Hill's Criteria of Causation (1965)
In 1965 Austin Bradford Hill detailed criteria for assessing evidence of causation. These guidelines are
sometimes referred to as the Bradford-Hill criteria, but this makes it seem like it is some sort of
checklist. For example, Phillips and Goodman (2004) note that they are often taught or referenced
as a checklist for assessing causality, despite this not being Hill's intention. Hill himself said "None
of my nine viewpoints can bring indisputable evidence for or against the cause-and-effect
hypothesis and none can be required sine qua non".
• Strength: A small association does not mean that there is not a causal effect, though the larger the
association, the more likely that it is causal
• Consistency: Consistent findings observed by different persons in different places with different
samples strengthens the likelihood of an effect.
• Specificity: Causation is likely if a very specific population at a specific site and disease with no other
likely explanation. The more specific an association between a factor and an effect is, the bigger the
probability of a causal relationship.
• Temporality: The effect has to occur after the cause (and if there is an expected delay between the
cause and expected effect, then the effect must occur after that delay).
• Biological gradient: Greater exposure should generally lead to greater incidence of the effect.
However, in some cases, the mere presence of the factor can trigger the effect. In other cases, an
inverse proportion is observed: greater exposure leads to lower incidence.
• Plausibility: A plausible mechanism between cause and effect is helpful (but Hill noted that
knowledge of the mechanism is limited by current knowledge).
• Coherence: Coherence between epidemiological and laboratory findings increases the likelihood of
an effect. However, Hill noted that "... lack of such [laboratory] evidence cannot nullify the
epidemiological effect on associations".
• Experiment: "Occasionally it is possible to appeal to experimental evidence".
• Analogy: The effect of similar factors may be considered.

9. Evan's Postulates (1976)
1. Prevalence of the disease should be significantly higher in those exposed to the
risk factor than those not.
2. Exposure to the risk factor should be more frequent among those with the
disease than those without.
3. In prospective studies, the incidence of the disease should be higher in those
exposed to the risk factor than those not.
4. The disease should follow exposure to the risk factor with a normal or log-normal
distribution of incubation periods.
5. A spectrum of host responses along a logical biological gradient from mild to
severe should follow exposure to the risk factor.
6. A measurable host response should follow exposure to the risk factor in those
lacking this response before exposure or should increase in those with this
response before exposure. This response should be infrequent in those not
exposed to the risk factor.
7. In experiments, the disease should occur more frequently in those exposed to
the risk factor than in controls not exposed.
8. Reduction or elimination of the risk factor should reduce the risk of the disease.
9. Modifying or preventing the host response should decrease or eliminate the
disease.
10. All findings should make biological and epidemiological sense.

10. References
• Fredericks DN, Relman DA (1996). "Sequence-based
identification of microbial pathogens: a reconsideration
of Koch's postulates". Clin Microbiol Rev 9 (1): 18–33.
• Hill, Austin Bradford (1965). "The environment and
disease: association or causation?". Proceedings of the
Royal Society of Medicine 58: 295–300. PMC 1898525.
PMID 14283879.
• Phillips, Carl V.; Karen J. Goodman (October 2004).
"The missed lessons of Sir Austin Bradford Hill".
Epidemiologic Perspectives and Innovations 1 (3): 3.