• By the end of the session students should;
• Be able to define and understand disease causation
• Understand the concepts of a causal relationship
• Understand the types of disease causation models (triad and causal pies)
• Effective disease control and prevention depends on
understanding the causes of the illness.
• In general terms, a cause is something that produces an
effect or brings about a result.
• important --- prevention, diagnosis and treatment..
• A cause---condition, characteristic or a combination of
factors playing an important role in producing a health
• Logically, a cause must precede an outcome
• the effect would not have occurred if the cause did not
• Another definition incorporates time factor;
• Rothman & Greenland, 1998-defined a cause as;
• an event, condition or characteristic that preceded a
disease without which the disease event either would not
have occurred at all or would not have occurred until
some later time.
• we expect that an increase in the level of a causal factor
in inhabitants will be accompanied by an increase in the
incidence of disease in that population
• if the causal factor can be eliminated or diminished, the
frequency of disease or its severity will decline.
• Basically what we imply is that:
• “if the person who developed disease Y had not been
exposed to factor X, then disease Y would not have
occurred. Therefore, X is a cause.”
• A cause is termed sufficient when it inevitably produces
or initiates an outcome (radiation or benzene-
• necessary if an outcome cannot develop in its absence
• Some diseases are caused completely by genetic factors in the
• alpha- and beta-thalassemias, sickle cell anemia, Marfan syndrome
• other causes of a disease interact with genetic factors in making
certain individuals more vulnerable than others Examples include
heart disease, high blood pressure, Alzheimer's disease, arthritis,
diabetes, cancer, and obesity.
TYPES OF CAUSAL RELATIONSHIPS
Necessary and sufficient
Necessary, but not sufficient
Sufficient, but not necessary
Neither sufficient nor necessary
Individual Assignment 1
• What is meant by causal relationship? Discuss with examples on the types of
1. Necessary and sufficient
2. Necessary, but not sufficient
3. Sufficient, but not necessary
4. Neither sufficient nor necessary
Due date: 25/11/22 printed copies, not more than 3 pages inclusive cover page
Factors in causation
• Necessary but they are rarely sufficient
• Predisposing factors:
• Predisposing Factors Definition Predisposing factors are the risk
factors that make a person more susceptible to developing a
• such as age, sex, or specific genetic traits that may result in a
poorly functioning immune system
• any characteristic of the environment that facilitates
• factors such as low income, poor nutrition, bad housing
and inadequate medical care may favour the development
• Precipitating factors
• factors that initiate or promote the onset of any illness,
disease, accident, or behavioral response
• such as exposure to a specific disease agent may be
associated with the onset of a disease
• Reinforcing factors
• Influencing Behavior Factors
• such as repeated exposure, environmental conditions and
unduly hard work may aggravate an established disease or
Causation of disease
• disease and other health events do not occur randomly in a
• occur in some members of the population than others-
risk factors that may not be distributed in population.
• Epidemiology concepts may be used to identify the factors
that place some members at greater risk than others
• A number of models have been developed as to why
• Among the simplest of these is the epidemiologic triad or
triangle, the traditional model for infectious disease
• consists of an external agent, a susceptible host, and
an environment that brings the host and agent together.
• Agent, host, and environmental factors interrelate in
a variety of complex ways to produce disease.
• Different diseases require different balances and
interactions of these three components.
• Can be a virus, bacterium, parasite, or other microbe
• presence of that agent alone is not always sufficient to cause
• A variety of factors influence whether exposure to an
organism will result in disease, including the organism's
pathogenicity (ability to cause disease) and dose.
• refers to the human who can get the disease.
• A variety of factors intrinsic to the host, sometimes called
risk factors, can influence an individual's exposure,
susceptibility, or response to a causative agent.
• Opportunities for exposure are often influenced by behaviors
such as sexual practices, hygiene, and other personal choices
as well as by age and sex.
• Susceptibility and response to an agent are influenced by
factors such as
• genetic composition
• immunologic status
• presence of disease or medications
• refers to extrinsic factors that affect the agent
• Climate change: Changes in temperature and rainfall
can alter the survival, distribution, and behavior of
insects and other species that can lead to changes in
• socioeconomic factors such as crowding, sanitation
• availability of health services.
• While the epidemiologic triad serves as a useful model for
many diseases, it has proven inadequate for some
• cardiovascular disease
• other diseases that appear to have multiple contributing
causes without a single necessary one
Component Cause (Causal Pies)
• Because the agent-host-environment model did not work
well for many non-infectious diseases, several other
models that attempt to account for the multifactorial
nature of causation have been proposed.
• One such model was proposed by Rothman in 1976, and
has come to be known as the Causal Pies.
• Most diseases are caused by the cumulative effect of
multiple causal components acting (“interacting”)
• Thus, a causal interaction occurs when two or causal
factors act together to bring about an effect.
• Causal interactions applies to both infectious and
• explains, for example, why two people exposed to the
same cold virus will not necessarily experience the same
outcome: one person may develop a cold while the other
person may experience no ill effects.
• Rothman’s (1976) causal pies helps clarify the
contribution of causal components in disease etiology.
• Figure 1.17 displays 3 causal mechanism for a disease.
• Let us assume these are the only 3 mechanisms that cause
• Wedges of each pie represent components of each causal
• corresponding to risk factors (we hope to identify and
diminish in the population)
• Each pie represents a sufficient causal mechanism,
defined as a set of factors that in combination makes
disease occurrence inevitable.
• Each casual component (wedge) plays an essential role in
a given causal mechanism (pie), and a specific disease may
result from a number of different causal mechanisms.
• A cause is said to be necessary when it is a component cause
member of every sufficient mechanism.
• In other words, the component cause is necessary if the
disease cannot occur in its absence.
• Component A is a necessary cause, since it is evident in all
possible disease mechanisms, and the disease cannot occur in
• For example, the tubercular bacillus Mycobacterium
tuberculosis is a necessary cause of tuberculosis
• However, it is not sufficient by itself to cause disease
• It is common for a person to harbor the Mycobacterium in
their body while remaining disease-free.
• Some individuals are not susceptible to tuberculosis; they are
• There are complementary factor that encourage disease to
• Include familial exposure, immunosuppression, genetic
susceptibility, poor nutrition, overcrowding, and high
environmental loads of the agent
• A sufficient cause is not usually a single factor, but often
comprises several components (multi-factorial causation)
• it is not necessary to identify all the components of a sufficient
cause before effective prevention can take place, since the removal
of one component may interfere with the action of the others
and thus prevent the disease or injury
• For example, cigarette smoking is one component of the sufficient cause of
• Smoking is not sufficient in itself to produce the disease: some people smoke
for 50 years without developing lung cancer.
• Other factors, are involved and genetic factors may play a role. However, the
cessation of smoking reduces the number of cases of lung cancer greatly in
a population even if the other component causes are not altered
• Each sufficient cause has a necessary cause as a component.
• For example, in a study of an outbreak of foodborne infection, it may be
found that chicken salad and creamy dessert were both sufficient causes of
• However, the ingestion of Salmonella bacteria is a necessary cause of this
• Similarly, there are different components in the causation of tuberculosis, but
the infection with Mycobacterium tuberculosis is a necessary cause
• Causal components that do not occur in every sufficient
mechanism yet are still essential for some of the causal
mechanisms are said to be contributing component
• For example, cigarette smoking is a contributing but not
necessary cause of lung cancer, since it contributes to the
cause of the (vast majority) lung cancer, but is not
necessary in every case
• Epidemiology - Leon Gordis 5th edition
• Basic epidemiology by R Bonita 2nd edition