I had a friend (roommate and ex-bethelite) who developed schizophrenia, which manifested itself after two suicide attempts. Ever since, I've been interested in the disease and try to learn about it as often as possible. I saw how the drugs disconnected my friend from some of his faculties (not being able to form coherent thoughts) and I saw the ineffective treatment professional therapy was providing him.
I'm convinced that there are multiple causes for it. And if genetics are involved, it may only mean that it may provide a propensity for its onset and not be a direct cause. Studies done with identical twins where one has it and not the other demonstrate that genetic inheritance is not always the culprit. Some researchers have suggested Epigenetics as a possible carrier for molecules that affect one person and not another with identical genes.
There are mild and extreme forms of it. My friend was seeing "monsters" on the wall and was hearing voices. Dr. Vilayanur S. Ramachandran, a behavioral neurologist that made several important discoveries while at U.C. San Diego, has demonstrated that an imbalance of chemistry, either too much or too little of one chemical that inhibits the activity of a part of the brain or makes it get out-of-had with activity, can send our thoughts into extreme states (thinking one is Jesus and can walk on water, that one understands everything in the universe, etc). Some of his patients experience this pathology after and because they suffer from seizures, other's because of damage due to an accident.
Frankly, I'm reluctant to make a distinction between biological or neurological or chemical causes. In the end, everything that happens inside the brain is due to transmissions of chemicals triggered reciprocally by and for electrical impulses (some chemicals trigger electrical impulses and some electrical impulses trigger the production of other chemicals). That they fail to work, may be due to genetic errors, to accidents or just an innate bad structure in the brain from physical forces or environmental triggers that prevent the normal from happening. Turrets is an interesting disease that shows the lack of control of specific parts of the brain while the sufferer still remains "functional" in most other respects. In a sense, that pathology is very similar in function to schizophrenia but with much less severe consequences.
It really pains me to see people who suffer with ailments like schizophrenia or Dementia (which slowly eats away your brain and robs your personality). I consider them by far the cruelest of diseases.