The fact that two genetically identical people can grow up together and one gets "schizophrenia" and one doesn't shows that it is caused by the chemical correlates of psychological events and the resulting change in physiology and function. If you can't wrap your mind around that it's not my problem.
Is that exclusive of physical events that result in a change of physiology and function? Such physical events such as childhood illness seem to be a factor in the development of schizophrenia. Or is your position that schizophrenia is only precipitated by psychological events that reveal an underlying predisposition?
Its my position that schizophrenia is not an entity rather a set of variables which when combined(in countless combinations) results in a common presentation. The massive amount of possible combinations of variables that will present as schizophrenia explain the rare occurrences of childhood onset and late life onset.
What i really want to stress is that schizophrenia requires delusion to exist, you can trace any psychotic episode back to one or many delusional beliefs. If you charted the development of the delusional thinking you could get a sense of the exponential kindling effect. The initial delusion might be very subtle and socially accepted but by the time it fully blooms its full blown psychotic. Psychological stress is the fuel for the fire and it feeds off itself. Thoughts have to be distorted In order for the other senses to become distorted which is why so much of the new research on schizophrenia is being done on sensory gating and cognitive enhancement.
All of the variables come together to set the perfect scene for the fire to be lit, genetics, parenting, culture, siblings, etc and yes it's possible a virus could change biochemistry or even damage parts of the brain, even head trauma could be a variable but in the end there is no possibility of psychosis in the absence of delusion.