From the 2012 Schizophrenia Commission report (subtitles added by me): 

"Nowadays, about 45% of people who receive a diagnosis of schizophrenia recover after one or more episodes, but about 20% show unremitting symptoms and disability and the remaining 35% show a mixed pattern with varying periods of remission and relapse (Barbato, 1998). 

There is no single cause but rather a range of factors combine to push an individual into psychosis. Genes contribute to vulnerability (Kim et al, 2011; Rees et al, 2011) and children who are born premature or suffer oxygen starvation at birth also have a higher risk. 

Heavy abuse of drugs such as amphetamines and cannabis is increasingly considered to be important (Di Forti et al, 2009; Casadio et al, 2011). 

A range of early socio-psychological adversities such as separation from a parent, being a migrant, growing up in a city, or being persistently bullied or abused, all increase risk of psychosis. Similarly, adverse life events and trauma can precipitate the illness. 

All the factors that increase risk of psychosis ultimately impact on brain dopamine levels (Di Forti et al,2007). For example, not only do recreational drugs increase dopamine levels but so does stress. High brain dopamine levels then lead a person to experience unusual perceptions and give excessive importance or “salience” to commonplace events (Van Os and Kapur, 2009). When people have such overwhelmingly strange experiences they try and find some explanation and this may lead them into delusional thinking.

Until recently it was thought that there was a clear distinction between people who had psychosis and the general population. However, numerous surveys have shown that up to 15% of the general population will experience hearing voices at some point in their life and 15-20% have regular paranoid thoughts (Tien, 1991; Freeman and Garety, 2006). 

There is therefore a continuum between those who have no psychotic symptoms and a sizable minority who have mild symptoms that cause them no trouble. A smaller proportion of these people are distressed by their symptoms and consult mental health services and are considered as having psychosis: many of those who are most distressed and disabled by their symptoms meet the conventional criteria for schizophrenia. Schizophrenia has blurred borders not only with normality but especially with bipolar disorder and with depression. 

This overlap is the reason why people may be given different diagnoses by different clinicians. Because of the dissatisfaction with the present system of categorical diagnosis, there have been repeated attempts to find an alternative. One is a system based on four symptom dimensions: psychotic symptoms (hallucinations and delusions); affective dysregulation (depression, mania and anxiety); negative symptoms (lack of motivation and withdrawal) and cognitive difficulties (Van Os et al, 2010)."

In 2011 Rethink Mental Illness set up an independent commission led by Professor Robin Murray, to look into how the needs of people diagnosed with schizophrenia are serviced in secondary care. They interviewed both experts in the field and people who have lived with this illness. Tere were 2500 respondents to an online survey. 

You can read the report in full here. 

Some 220.000 people in England are diagnosed with Schizophrenia. These people are cared by a system that the independent commission found was "broken and demoralised" and "does not deliver the quality of treatment that is needed for people to recover".

Some of the problems identified by the commission:

The commission also heard that "good care delivered by kind, compassionate practitioners can make all the difference.(...) Time and time again we heard of a transformation whereby an apparently downward course was reversed by a nurse, doctor, peer or therapist who tool the time to listen and understand."

Some facts about severe mental illness