“Schizophrenia is a mental disorder characterized by abnormal social behavior and failure to understand what is real. Common symptoms include false beliefs, unclear or confused thinking, auditory and visual hallucinations, reduced social engagement and emotional expression, and a lack of motivation.” Source: Wikipedia
Current estimates are that at any one time there are around 20 Million sufferers of schizophrenia worldwide. It's known that a variety of situations can trigger schizophrenia in vulnerable individuals - stress, drug use, infections, genetic factors etc - but the pathways (and commonality, if there is any) are not yet discovered.
“The past several decades have […] brought tremendous advances in neuropharmacology that have extended our understanding of schizophrenia at both a neurochemical and neuroanatomical level. However, despite the advent of sophisticated analysis and imaging tools, or perhaps even because of them, our ability to synthesize a coherent model of schizophrenia remains as elusive as ever.” Source: J Psychiatry Neurosci. 2001 Nov; 26(5): 395–410.
Currently, medication can help relieve the severity of the symptoms (rather than addressing the causes - therefore designing effective medications is highly problematic since the causes of the disorder have yet to be accurately identified.)