Originally Posted by kaneohe
I know you're not supposed to open attachments from links in e-mail
unless from trusted sources (and even then it could be bad guys masquerading as trusted sources) but can just opening an e-mail message
Sadly, yes. If you have a current antivirus setup running, make sure it is set to scan e-mails as well as downloads from your web browser.
The most common trick with e-mail currently is to include an image or PDF file that is constructed to cause what's called a 'buffer overrun', that is, it's designed to tickle an existing bug in image handling software to get your computer to execute some bit of code.
A close runner up is HTML mail, where opening the message may cause a new connection to the Internet to download some additional content, such as images, to complete the mail page. That content is corrupted in a way to exercise a bug as above.
Manufacturers of software are pretty good about patching these sort of problems with software updates. The attacks are usually aimed at somewhat older systems not likely to be patched.