This link Anna http://www.schizophrenia.com/szfacts
should take you to a site that examines some of the incredibly complex causes of schizophrenia and looks at its incidence..
As you will see the biggest factor is genetic predipostition...but a whole range of factors , social and even environmental come into play.
Schizophrenia is found in every culture, and is well described in the literature even before the name was coined.
Two people can inherit the gene/s and one will develop the full blown condition and one not, depending on the social and envionment factors alluded to above.
Drug use is just one factor that can precipitate schizophrenia but there are many many others which are beyond the conscious choice of the person with the condition..