Most computer users are running computers with a Microsoft Windows operating system. That, all by itself, makes a computer susceptible to a virus or other dangerous or malicious software ("malware").
I'm not trashing Microsoft; I'm stating a fact. Microsoft Windows is so prevalent that it's a target for those that create viruses or other malicious software.
It's common for many computer users to get a virus. The most common reason is a lack of proper protection. That involves not having good, up-to-date security software. It also involves careful computer habits.