For me, I've been diagnosed with ADHD & schizoaffective disorder.
It's weird, as there seems to be an overlapping of symptoms in many mental illnesses. One mental illness can present itself in various ways. Even brain tumours sometimes manifest themselves in ways that match the symptoms of psychosis.
I think doctor's and medical scientists need to look into this further. Which they have, to an extent, but, psychiatric treatment hasn't seemed to catch up.
The DSM-5 is more akin to a guidebook if anything IMHO.
A concept called the “p factor” attempts to explain why psychiatric disorders cannot be clearly separated
www.scientificamerican.com