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 later. Time went on, viruses multiplied. They all were all alike in a sense, tried to get to RAM, stuck to files and sectors, periodically killing files, diskettes and hard disks. One of the first "revelations" was the "Frodo.4096" virus, which is far as I know was the first invisible virus (Stealth). This virus intercepted INT 21h, and during DOS calls to the infected files it changed the information so that the file appeared to the user uninfected. But this was just an overhead over MS-DOS. In less than a year electronic bugs attacked the DOS kernel ("Beast.512" Stealth virus). The idea of in visibility continued to bear its fruits: in summer of 1991 there was a plague of "Dir_II". "Yeah!", said everyone who dug into it.
But it was pretty easy to fight the Stealth ones: once you clean RAM, you may stop worrying and just search for the beast and cure it to your hearts content. Other, self encrypting viruses, sometimes appearing in software collections, were more troublesome. This is because to identify and delete them it was necessary to write special subroutines, debug them. But then nobody paid attention to it, until ... Until the new generation of viruses came, those called polymorphic viruses. These viruses use another approach to invisibility: they encrypt themselves (in most cases), and to decrypt themselves later they use commands which may and may not be repeated in different infected files. Polymorphism - Viral Mutation
The first polymorphic virus called "Chameleon" became known in the early '90s, but the problem with polymorphic viruses became really serious only a year after that, in April 1991, with the worldwide epidemic of the polymorphic virus "Tequila" (as far as I know Russia was untouched by the epidemic; the first epidemic in Russia, caused by a polymorphic virus, happened as late as in 1994, in three years, the virus was called "Phantom1").
The idea of self encrypting polymorphic viruses gained popularity and brought to life generators...