We have seen a tremendous rise in the amount of computers infected with malware over the past few years.
Malware viruses are downloaded to your computer when you open suspicious emails. Sometimes these emails can even appear to be from someone you know whose email account has been hacked, or they can be downloaded to your computer simply by clicking on a link in a message sent via Facebook.
Do not click on unknown links in emails.
By clicking on these links it downloads a virus to your computer that downloads information that you type on your computer to the virus creator. This can include usernames and passwords to the sites you visit, and even your credit card numbers if you shop online.
The best defense against this is making sure you are running a good anti-virus software suite on your computer and that its definitions are up to date.