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Originally Posted by boagie  In other words dis-ease is not the act of a healthy body, it is a reaction to a foreign object and/or substance, which changes the course of one's constitution in health. |
Speaking as a well-entrenched insider in the medical profession, I think you've got it half right. There are both intrinsic and extrinsic things that can affect a given disease.
Take pneumonia. Pneumonia is an infection of the lungs (typically a bacterial infection). The bacteria that most often produce pneumonia have toxins that cause tissue injury, that provoke inflammation, and that ultimately produce symptoms. So here it seems that the bacteria is a foreign object or substance, right? And you're correct that
much of the damage caused by pneumonia is caused by the body's inflammatory response.
But now consider that there are intrinsic host factors that affect susceptibility to pneumonia and risk of complications with it. The elderly are FAR more susceptible to pneumonia than people who are young and healthy. People with cystic fibrosis (which is a genetic disease) have pulmonary infections all the time. These are unmodifiable risk factors. There are many other extrinsic things that will alter a host's intrinsic susceptibility -- for instance smoking (extrinsic) will lead to a host who is
intrinsically more susceptible to infection by pneumonia pathogens. If someone develops cancer (sometimes intrinsic, sometimes extrinsic), the chemotherapy (extrinsic) will lead to a host with an intrinsically weak immune system that is more susceptible.
So diseases can't cleanly be thought of as intrinsic or extrinsic. But to be sure, very often there are external factors that either cause injury or that provoke an injurious response by the body, and there are tons of examples.