What are the most causes of viruses on computer devices?

[Stardom22]

I oftenly saw that notifications on my old desk top that said, your computer is running slowly it seems to be infected with some computer viruses. So l really wonder what might be the cause of the computer viruses?

 

[eLdavis]

Visiting unprotected sites and downloading corrupted files can be a major cause of getting your computer corrupted with virus.