One of the easiest ways to get a virus is by opening an attachment that you weren't expecting. Most of my customers stories start with "so I got this email and it came from me to me, so I got curious and opened the attachment." OR "UPS sent me a message about a package I never sent and said it couldn't be delivered so I opened the attachment per the instructions and now my files are gone".
The general rule of thumb in today's email world is if it looks even a little bit odd or you get an email you weren't expecting with a questionable attachment don't open it! The minute you download an attachment it is on your computer and can execute code to cause harm. Make sure you use an email provider with good spam and virus protection and make sure you have an up to date virus and malware protection program.
Again if it looks fishy don't mess with it! Always error on the side of caution.
Click here to learn more about how to spot a phishing attempt.