Although there is no inherent reason a virus has to be malevolent (and the first ones reportedly were not), it was certainly easy enough to write viruses that could erase hard drives or do other serious damage.
In the early 1970s a virus reportedly infected the ARPANet — a virus which used a modem to dial out and infect other computers. It caused no damage. [I can remember stories about answering the phone and hearing a computer modem at the other end of the connection — presumably a virus was dialing out and looking for other computers to infect. I don’t know that it ever happened to me.]
The first virus to spread massively was Elk_Cloner, which infected Apple II computers in the 1980s. [I never saw this one myself, though I was actively developing for the Apple II at the time.] It was spread from system to system by the sharing of infected floppy disks.
Like a real-world virus, some of the first computer viruses could inject or attach themselves to an existing program and thus would spread themselves by executing (and infecting other programs) when the legitimate (but infected) program was started up. Such a virus would usually search the computer’s disk, finding each application program in turn, and rewriting or adding code to the application programs making them capable of further infecting any computer they came in contact with. This kind of infection could be spread by sharing program on floppy disks. And sometimes a virus would even become attached to a commercial program before it was distributed to stores.
See the Wikipedia Timeline of Notable Computer Viruses and Worms
An early (2000) history of viruses on microcomputers
 Interestingly enough, viruses sometimes didn’t seem too interested in real death and destruction. Even those that would “delete” the hard drive really just destroyed the disk’s directory, and recovery programs still stood a pretty good chance of recovering most or all of the data on an “erased” hard drive.
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