Question

What is a main cause of a virus on a computer

Answer 1

one of the ideal methods by which viruses spread is through emails-opening the attachment in the email, visiting an infected website, clicking on an executable file or or viewing an infected advertisement can cause the virus to spread while connecting with already infected removable storage devices, such as USB drivers.

Answer 2

The main cause of computer viruses is putting an infected USB flash drive into your computer, opening an infected email, going to websites that are dangerous, and downloading infected files Some viruses disguising them self's as pop-ups. Files that could have downloaded themselves while you are on theses dangerous websites are viruses as well.
Be carful and make sure you don't do anything to deal with these! ;)