As the initial source of infection in a certain geographical area maybe but as far as I remember most viruses (specially boot sector ones) spread through floppy disk sharing, first from people to people inside companies, from company computer to personal computer at home, from friend to friend personal computer and then from personal computer to company computer.

Much like the spread of HIV back in the late 80's and early 90's most people didn't really understand how exactly computers programs worked and didn't follow IT guidelines on how to avoid getting infected. The number of infections was naturally limited by the small number of people at risk: computer users.

But as the availability of computers and the number of useful applications increased so did the volume of infections being spread through the same bad habits: floppy sharing without protection, and by that I mean the read only lock.

And, instead of the ideal (but very hard) way to eradicate the problem (informatic prophylaxis and education for users) the industry "solved" the problem by creating the antivirus and accepting an occasional infection as something unavoidable.

Then the World Wide Web exploded, creating a frictionless media for the spread of these infections and here we are.