All disease stems from cells malfunctioning. Why do cells malfunction? 1) Not getting enough of something it needs, or 2) getting too much of something it doesn’t need.
Every disease can be traced back to some kind of deficiency or toxicity that causes cells to not function properly, and for the rest of the pathology lectures, we will learn about specific examples. It is important to note that humans evolved to be well adapted to an environment that was very different than our modern world. A majority of modern diseases result from a mismatch between what our cells need to be healthy and what they are exposed to via our behavior, diet, and environment. Medicine very often suppresses symptoms without addressing the underlying deficiency or toxicity, and can actually perpetuate chronic disease. This idea is well developed in the book The Story of the Human Body by Daniel E. Lieberman.
*I just want to clarify that this is a simplified explanation for pathology only. The actual manifestation of disease is infinitely complex, reflecting the interaction between the mind, body, and socioeconomic determinants of health!