DNA and Disease
DNA is fundamental in building a functioning body. It is also responsible for maintaining and defending the body against "the thousand natural shocks flesh is heir to". It is perhaps unsurprising that mutations in DNA cause disease.
There are around 200 different types of cell in the human body. All cells (except red blood cells) contain the entire complement of human DNA; however, some areas of DNA are active and generate proteins only in specific cells. Therefore, mutations in these areas will only affect these cells. Alternatively, some areas of DNA are active in all cells but different cells are susceptible to different environmental stress and this exaggerates the affect of the mutation in these cells. For example, substantia nigra nerve cells are lost in Parkinson's disease; these cells are under high oxidative stress and any mutations that cause extra stress will preferentially affect the substantia nigra.
DNA ("nature") and the environment ("nurture") are likely to play some part in all disease; some diseases are weighted towards DNA as cause (e.g. Huntington 's disease) while others are caused primarily by our surroundings (e.g. liver disease due to alcohol consumption). Parkinson's disease is likely to be somewhere in the middle with many subtle genetic factors interacting with the environment to cause loss of nerve cells.
See Nature 421 395-453