Yes, most of the computer virus have security threat to the infected computer. If your computer has been infected with virus, you should remove it quickly.
Almost any activity on the Internet can cause your computer to be infected by a virus. The most common are emails, social media (such as forums, Facebook, etc), pictures, and downloads.
The Macro virus is the most common type of computer virus. The ILOVEYOU virus is one example of this type of virus which hides in a document or spreadsheet.
contact with an already infected person/animal.
Most likely downloading and executing an infected file.
Anti virus software is useful, because it protects your computer from getting infected by a computer virus. Moreover, most software will reduce the damage a virus can do to your computer. Mind you, the protection is not 100 %. New viruses come out every minute, so your computer may still be infected before the anti virus software recognizes the virus and is able to remove it.
pop-ups, infected websites.
Yes it's one of the most common ways-don't open links on anything suspicious or scan prior to opening
backup your most important files, and then wipe the hard-drive clean.
downloads or e-mails