Bad guys are getting smarter by the month. They now send you emails that your antivirus and spam filters do not catch. It goes like this: The email has an attachment that you are tricked into opening. In the attachment is a link that they try to make you click on. The link goes to a malicious website and will infect your computer.
Do not open attachments you did not ask for. When you get an attachment, verify if that person sent it to you and why. If in doubt, throw it out. Always Think Before You Click.