There are many potential sources of malicious infection, including floppy disks, e-mail messages, shared files, and downloaded documents.
Whether virus, worm, Trojan horse, or spyware, most malicious code is trying to get onto your computer by hiding itself somewhere that looks innocent. For example, it could be in an e-mail attachment you receive, some software you download from the Internet, a Web site you visit, a shared file or network, a floppy disk you share with someone, or even inside a Microsoft Office document. Basically, any information that comes from another computer could pose a risk.
Note There will be a few notable exceptions where there's nothing hidden about it and the attack is blatant.
If you kept your computer in a sealed room, never linked it to a network or the Internet, and never inserted a CD or floppy disk, then you would remain safe. But that would make your computer practically useless. For example, if you weren't connected to the Internet, you wouldn't be able to take this training course.
As well as being wary of information coming into your computer, you should be careful when visiting Web sites that request information from you and ensure that you are on a genuine site before entering any personal data. For instance, you should make sure you're on the real site of your financial institution and not one that is faked to look like it. Also note that when you visit any Web site you're actually downloading files behind the scenes, so make sure the sites you visit are trustworthy.