I have had an instance of Crypto locker ransomware today on a client's site, would have been catastrophic if proper backup and disaster recovery practices had not been in place.
Be careful what emails you open, and certainly don't click on any links or open any email attachment files from suspect emails or unexpected emails from organisations such as:
Australia post
Any of the big banks
DHL
Fedex
ATO
Plus others.
Because Crypto locker isn't a virus as such it won't be detected by most antivirus software. This being the case you need to use care in evaluating the legitimacy of any email. Things to look out for are poorly formatted emails, spelling mistakes etc you can also hover your mouse over any links in outlook and get a popup "tip" of where those links point to.
If in doubt call me before opening, viewing clicking or downloading.