Internet viruses came with the internet. But during the early years:

Pop ups.

Yeah it was mostly pop ups and occasionally links in emails.

Pop ups with porn were especially used for this because men are dumb and will click on titties.

People used to share the viruses because the virus would lock their computer and say something like “you must email this to 10 people to get back access”. Or something like that. I forget exactly but that used to be a thing.

Your own family and friends would send you viruses.