Where a PC virus comes from
Viruses are usually hidden in a commonly used program, such as a game or PDF viewer, or you may receive an infected file attached to an email or from another file you downloaded from the Internet. As soon as you interact with the file (run the program, click on an attachment or open the file), the virus runs automatically. The code can then copy itself to other files and make changes on your computer.
How to recognize a virus
Your Internet connecting might be slow or non-existent. It's common for your antivirus and/or firewall protection to disappear or be disabled. Computers infected by viruses are even capable of acting on their own, executing actions without you knowing.