Are you familiar with phishing?
It is when someone out there in cyberspace gets hold of your online information, and sends emails with viruses or spam from or to your account.
It could just be an annoying prank, but a lot of the time phishing is used by people who want to steal your personal information and bank account numbers. A talented phisher can hack into your system, find out where you do your banking online, and send you a mock email that really looks like it's from your bank. The email might claim that there's a problem with your account and that you need to click through to the website and enter in your user name and password. However if you do, of course, the website you're led to isn't the real bank site, but rather a clever mock-up meant to fool you.
Around half of email users in the US get a phishing email at least once a day. How can you tell the difference between a real email from your bank and a fake one? It can be difficult, but one rule of thumb is that a real bank would never send an email asking for your account information. If you're not sure, call the bank and ask what's going on. Chances are they'll tell you they didn't send anything.