Many factors are known to be important in disease causation and these can include an individuals age, gender, genetic make-up and environmental factors that they have been exposed to, such as infectious agents or chemicals.
Not all individuals exposed to the same level of a risk factor, for the same length of time, will develop a particular disease. For example not everyone exposed to the same level of tobacco smoke for the same length of time, will develop mouth cancer. In other words some individuals are more susceptible while others are less susceptible.
This variability may be due to an individuals inherited genetic make-up and/or some other potentiating risk factor. Very rarely the susceptibility to develop a disease is entirely genetic or purely environmental; for most it is a combination of these factors.
Some diseases that are thought to have an inherited or genetic component are seen to cluster in families and affect more than one generation, but without a clear pattern of inheritance as for monogenic diseases. Examples of this include early onset cardiovascular disease and Type II diabetes. Identifying genetic variants that increase or decrease an individuals susceptibility to disease can potentially lead to the targeting of preventive measures at those who are at greatest risk. This may also give valuable insights into the underlying molecular processes at a cellular level that are important in disease causation, opening the way for new and novel therapies to improve the outcomes of those with disease or who are at risk of disease.
There are a number of different types of epidemiological study that are available to look at the degree of genetic influence in different diseases.