This is one of the biggest problems with Identity Theft. You can take every precaution yourself and still have your identity stolen because someone you did business with was careless.
You filled out a job application or a loan application, and the paper form is left unguarded in an office. Your data is on a computer, and it is stolen as in the Flagstar case or hacked into.
Medical records are another popular (but little known) area of identity theft. Your identity stolen there can result in a life or death situation.