One of the first viruses to gain widespread attention was "The Brain." Two brothers from Lahore, Pakistan had written a heart monitoring program, and realized it was being copied and distributed illegally. So in 1986, they released a version of their software (as well as other pirated bootleg titles) that contained "The Brain," kind of as an experiment.
Essentially it would copy the boot sector of the floppy (or hard drive) to another part of the disk, and mark is as "bad." It would then alter the disk label to "©Brain" and replace the boot sector with a copy of the virus.
The boot sector also contained the following text:
Welcome to the Dungeon (c) 1986 Basie & Amends (pvt) Ltd VIRUS_SHOE RECORD V9.0 Dedicated
to the dynamic memories of millions of viruses who are no longer with us today - Thanks
GOODNESS!! BEWARE OF THE er..VIRUS : this program is catching program follows after these
Welcome to the Dungeon © 1986 Brain & Amjads (pvt). BRAIN COMPUTER SERVICES 730 IZANAMI
BLOCK ALLAMA IQBAL TOWN LAHORE-PAKISTAN PHONE: 430791,443248,280530.
Beware of this VIRUS.... Contact us for vaccination...
In addition to that, "The Brain" also caused hard disks to slow over time, and even led to data loss. From Going Viral: How Two Pakistani Brothers Created the First PC Virus:
Shortly after the University of Delaware outbreak, Brain began popping up at other universities, and then at newspapers. The New York Times reported that a “rogue computer program” had hit the Providence Journal-Bulletin, though the “damage was limited to one reporter losing several months of work contained on a floppy disk.”
What's interesting about "The Brain," was that it didn't infect other computers over the internet. It relied on users to spread it inadvertently via floppy disks. From the same article mentioned/quoted above, the brothers were quite shocked at the size of the response:
Basit and Amjad began receiving calls from all over the world. They were as surprised as anyone that their little experiment had traveled so far. After all, unlike today’s computer viruses, which spread at lightning speed, Brain had to transmit itself the old-fashioned way—through human carriers toting around 5.25-inch floppy discs.
And as much as I hate referencing things on Wikipedia, here's a link to the Wikipedia article on "The Brain": Brain (computer virus)