You receive an email from a well-meaning contact warning you of an impending infection. There is a 100% probability that the threat your friend is warning you about is a hoax, or a actual threat that occurred years ago. The warning email from your friend often carries a real virus!
These threats work by depending on a recipient who will open an attachment. It takes a very small number of these mistakes. 1 in a 1000 will do just fine. The infection spreads exponentially and it does it in a matter of a few hours.
The good sense not to open an attachment or click on a link is your best protection. Attachments can be dangerous – even if it is from someone you know and trust. Never open an attachment or click on a link unless you are expecting it. If it is from someone you know, ask them first if they intended to send it to you. It could be that their computer is infected and has sent this.