At a general level, in my view, the biggest advances in medical science in the past few decades have been those that have allowed us to reclassify diseases and medical conditions by medical causes that were previously unknown.
Some early waves of this process identified conditions caused by particular virus or bacterium or parasite, conditions caused by particular single nutrient dietary deficiencies, and conditions caused by simple chromosomal genetic issues (like Down's syndrome or Hemophilia A).
But, since then, a number of important classes of conditions have been identified and explored at much greater depth.
For example, we are getting a much better grip on the importance of mitochondrial DNA, in addition to the historically center stage player, nuclear DNA, which is implicated among other things in many metabolic conditions.
We are identifying conditions associated with symbiont bacteria, particularly in the gut and in the mouth. There are now cases where a "gut bacteria transplant" can change how someone metabolizes food, for example.
We are starting to understand several classes of auto-immune diseases.
We are starting to recognize prions as disease agents in diseases like Alzheimer's and Mad Cow disease, as alternatives to agents like viruses and bacteria.
We are getting a better handle on age related genetic mutations.
We are starting to better understand a variety of medical conditions associated with the environments we encounter in the womb.
We are learning that estrogen impacts two different kinds of receptors in the body, either of which can malfunction. We are also starting to understand how some of the less famous hormones affect us.
We are discovering that people have environmentally responsive epigenetic instructions in their bodies layered upon the core nuclear DNA that someone receives at birth.
We are starting to split out mental health conditions with very important and well understood environmental components that constitute immediate causes (e.g. PTSD and many cases of clinical depression), from those that emerge early in life and have less direct environmental causes (e.g. autism and psychopathy) from those that emerge relatively late in life and appear to have strong hereditary components (e.g. schizophrenia and bipolar disorder). We are also starting to understand some mental health (and non-mental health) conditions that have genetic components but only manifest in the presence of particular "triggers." For example, some people are hereditarily prone to being screwed up emotionally by child abuse and neglect, while other people, sometimes siblings, have more of a hereditary ability to resist the negative influences of these conditions.
We are starting to identify the chemical pathways and brain components associated with particular mental conditions -- distinguishing, for example, conditions that act on the dopamine or serotonin systems, and conditions associated with the amygdala. Understanding these brain systems also allows us to develop biologically based descriptions of dimensions of personality and mental difference between people.
The hope, of course, is that we can better diagnose conditions that present in a similar way but have different causes, and that by understanding causes better that we can better know how to deal with these conditions.