Things do not just happen. And neither do diseases. Why a person who is healthy go suddenly sick and another one not? Because there are a number of factors that are distributed in the population that make some people more at risk of disease than others.
This has drawn the attention of epidemiologists forever, hence they have tried to describe models to illustrate the relationship between patients and diseases, to try to understand the causes of disease.
Models of causality
The simplest model is formed by a wicked triangle involving three elements: the agent, host and environment. These three components interact in a more or less complex way to lead to the onset of disease.
This model was originally devised for infectious diseases, and the agent in question is the microorganism that can cause disease. For example, plasmodium causes malaria. Naturally, different diseases depend on the presence of different agents in addition to various interactions among agent, host and environment.
The host is the poor guy who gets the disease. Not all guests are equally susceptible to the disease. There may be a number of risk factors intrinsic to the host that increase or decrease the risk of disease, even though he gets in contact with the agent. In our example about malaria, subjects with sickle cell disease have a lower risk of malaria than carriers of a normal hemoglobin.
The third side of the triangle consists of the environment, whose main function is to bring into contact the agent and the host to produce the disease. If there are no mosquitoes that transmit the parasite, there will be no cases of malaria, no matter how much plasmodium there is in the pond.
The problem with this stylish model is that it explains much better infectious diseases than other types of diseases. Take the example of smoking and lung cancer. We all know that smoking causes cancer, but not all smokers suffer from it and not all cancer patients are smokers. It seems, then, that this matter of causality is something more complex.
And this is where we run into the pie. Imagine all the causes that may be related to a disease like pieces of a pie. If the pie has the necessary portions, the disease occurs. Otherwise, the disease does not occur.
Components of causality
But not all the pieces of the pie are equally important. Thus, we recognize several categories of causes. A portion may be a component cause, that is each of the individual components that can lead to disease.
One component cause may consist of factor related to the agent, to the host or to the environment, but on its own it might not be enough for the disease to occur. One may be exposed to the agent for years and not develop the disease. For example, exposure to a virus does not guarantee the infection if the guest does not have, in addition, other factors of susceptibility.
On the other hand, we have the necessary causes. Without them, the disease does not occur even though there are various component causes. Consider the example of an infection by an opportunistic germ in an immunocompromised host. The pathogen is not able to cause the disease if the immune system is unharmed, so the immunosuppression would be a necessary cause for the infection to occur.
Finally, we have the biggest piece of the pie, de sufficient cause. This piece alone completes the pie. This single cause leads to disease without any more necessary or components causes. A disease may have one or more sufficient causes. Finally, we can think about a sufficient cause as the concurrence of several components causes.
To conclude, we can loop the loop of types of causes combining them as follows:
- Necessary but not sufficient: without the casus there can’t be disease, but its presence does not guarantee it. For example, papilloma virus and cervical cancer.
Not necessary but sufficient: it produces the disease, but it can be also caused by other factor that may act on their own. For example, think of a tumor that can be produced by radiation, but also by chemical carcinogens.
Not necessary and not sufficient: none is essential for the diseases and none causes it alone. For example, all risk factors for diabetes or heart disease.
And here we leave triangles and pies, but not before some thoughts. Everything we have said about causes of diseases we could have said about protective factors against the development of disease. For example, quitting smoking could be a component factor for preventing lung cancer. But that is another story…