Per my hypothetical patient above, if Mrs. X winds up with a STD, that information is in her medical chart. Down the road she or a member of her family applies for a loan, job, scholarship, security clearance, etc., etc., etc. Anyone along the way can access the record and add it to other data bases.
Thus, my friends know to never send me an email that includes the addresses of others, i.e., if they just must send the latest corny joke, they know to send it to Bcc "Undisclosed-Recipient" or suffer my wrath. There are those who work for spammers who have software that most easily harvests email addresses from those passed around the world urban legends, jokes, etc.