What can cause schizophrenia?

Simply... A combination of genetic vulnerability and exposure to stress.
Multifactorial. As with many medical conditions the causes of schizophrenia are not fully know. It is believed they are polygenetic, with multiple genes interacting resulting in some degree of vulnerability or resistance to the condition. This biology then interacts with environmental factors, both positive and negative which can have different effects based upon when in life or how many/often they occur.