They are actually advising users to shutdown TeamViewer after they are done using it. Also, they suggest enabling 2FA and not reusing a password from anywhere else.

TeamViewer Support:
[Linked Image from s33.postimg.org]

I have a theory as to how they pulled it off, but I'm checking to see if it's consistent with the events.

Another good idea is to check if your email has been compromised by the recent breaches (LinkedIn, etc): https://haveibeenpwned.com/