Attachments sent along with email aren't a threat to your system if you don't download them. However, if you choose to download an attachment by either opening it or saving it to your computer, your computer does then become vulnerable to computer viruses. (The same is true of all files you download to your computer, whether email attachments or not.)

Fortunately, Yahoo! Mail automatically scans all incoming and outgoing attachments for viruses using Norton AntiVirus, and alerts you to any viruses it detects.

We'd recommend that you do not download any attachments or other files from a source you don't know and trust.