A Bit of Archeology
There are lots and lots of opinions on the date of birth of the first computer virus. I know for sure just that there were no viruses on the Babbidge machine, but the Univac 1108 and IBM 360/370 already had them ("Pervading Animal" and "Christmas tree"). Therefore the first virus was born in the very beginning of 1970s or even in the end of 1960s, although nobody was calling it a virus then. And with that consider the topic of the extinct fossil species closed.
Let's talk of the latest history: "Brain", "Vienna", "Cascade", etc. Those who started using IBM PCs as far as in mid-80s might still remember the total epidemic of these viruses in 1987-1989. Letters were dropping from displays, crowds of users rushing towards monitor service people (unlike of these days, when hard disk drives die from old age but yet some unknown modern viruses are to blame). Their computers started playing a hymn called "Yankee Doodle", but by then people were already clever, and nobody tried to fix their speakers - very soon it became clear that this problem wasn't with the hardware, it was a virus, and not even a single one, more like a dozen.
And so viruses started infecting files. The "Brain" virus and bouncing ball of the "Ping-pong" virus marked the victory of viruses over the boot sector. IBM PC users of course didn't like all that at all. And so there appeared antidotes. Which was the first? I don't know, there were many of them. Only few of them are still alive, and all of these anti-viruses did grow from single project up to the major software companies playing big roles on the software market.
There is also an notable difference in conquering different countries by viruses. The first vastly spread virus in the West was a bootable one called "Brain", the "Vienna" and "Cascade" file viruses appeared later. Unlike that in East Europe and Russia file viruses came first followed by bootable ones a year