Muggle Protection Act

[mhershey2001]

No: HPFGUIDX 72629

I am new here, so maybe this has been discussed, but doesn't the 
whole Wizard plan to keep their world from the Muggles just fall 
apart when you realize that there are very few pureblood families 
left, so there must be at least dozens, if not more, Muggle families 
who are related to Wizards and thus know all about the Wizarding 
world?  After all, at the beginning of PoA (I think), Hermione's 
parents actually come to Diagon Alley with her and kids at Hogwarts 
are always talking about their Muggle parents (Creevy, Dean Thomas, 
etc.).  Further, part of the charges against Harry for using magic 
against the Dementors was that he performed the Patronus Charm in 
front of a Muggle - yet, Dudley obviously already knew Harry was a 
Wizard.  So why all the attempts at secrecy, and how does the wizard 
world stay a secret?

Mhershey