What is the pathophysiology of diabetes mellitus type 1?

Beta cell death: Type I diabetes ensues after significant beta cell death which depletes insulin production and causes quantitative insulin deficiency. What causes beta cell death remains unclear. Genetics plays some role, possible infectious agents (viruses) but no definite cause has been found to date.

Diabetes Type 1: In Diabetes Type 1, the islet cells on the pancreas are not able to produce insulin any more. For some reason the immune system typically attacks the islet cells and destroys them. In some cases it is thought to be caused initially by an immune response to cow's milk.

Complex : This is an entire course in med school, and a lifetime of study for the specialists. Essentially, the pancreas is unable to meet the body's requirements for insulin production, whether caused by disease, trauma, toxicity. Read: http://www.nlm.nih.gov/medlineplus/diabetestype1.html