A computer virus is a self-replicating computer program that spreads by inserting copies of itself into other executable code or documents. Viruses nest in files or in parts of the system memory and spread over networks, emails, Filesharing programs and media as for example disks, CD-ROMs or USB-Sticks. The insertion of a virus into the program is termed as an "infection", and the infected file, or executable code that is not part of a file, is called a "host". Viruses are one of the several types of malicious software or malware. In common parlance, the term virus is often extended to refer to worms, trojan horses and other sorts of malware.

Nevertheless, a virus reaches your computer in many different ways, sometimes even with your permission, though mostly while hiding behind other programs. Before a Virus is installed, you normally have to click an object on the Internet or to open a program. In the following you can see a listing of all the ways a Virus primarily reaches your PC:

More than 90% of all computers connected to the Internet are infected with viruses nowadays. The following behaviors suggest that your PC is infected with a virus: