In that case, it probably is not a virus.  It is probably malware.  At some point in the past, you may have installed a program that you thought was legitimate, and that program was probably designed to steal your bitcoins.

Have you ever installed any pirated software on your computer?
Have you installed wallets for any altcoins on your computer?

Both of those are very common ways to unknowingly install malware.

The other possibility is that you downloaded software from a phishing site without realizing it.  Some phishing sites can look exactly like the real site.  When was the last time that you downloaded some software from a website?


Malware on your computer can look for an installed wallet. It can then capture your password as you type it.  Once it has your wallet and your password, it can access your private keys and spend your bitcoins.