The Brain virus is an early virus targeting the IBM PC / MS DOS platform.
Sometimes, it is claimed that one of the virus' purpose was to prevent the copying of some software that the virus was distributed with.
For example from the virus' wikipedia page (unfortunately unsourced):
This program was originally used to track a heart monitoring program for the IBM PC, and people were distributing illicit copies of the disks. This tracking program was supposed to stop and track illegal copies of the disk,[...]
For example a news report from Networkworld, based on an interview of the authors:
"[...] We wrote it for our medical software to be protected from piracy [...]"
The earliest source I can find in regards to the motivation of the original authors is from the Times magazine (1988):
Then why infect American buyers? "Because you are pirating," says Basit. "You must be punished."
Interestingly, there is no mention of copy protection in the sense of preventing a copy, just the "punishing" aspect.
In a later interview with the original authors, the authors claim that their motivation was to experiment with the then new platform and figure out its security aspects.
I have taken a look into an old technical report about the virus and it seems to me that the virus copies itself indiscriminately to each suitable floppy disk that is read from (for whatever purpose). So the virus would spread to each an every disk regardless of whether an (illicit) copy was made or not.
From the technical report I cannot fathom how copy prevention by a technical measure would have happened. I understand that having all your disks infected would be a hassle and deterrent.
Did the Brain virus technically prevent copying of the software it was distributed with and if yes, how so?
Did the Brain virus only proliferate with illicit copies of the software it was protecting?
wikibooks provides disassembly of a variant of the Brain virus. Although not completely polished, write requests will go through unhindered (as far as i can tell, see the int13 label)
A copy of an early advisory also claims:
No known intentional damage. Unintentional damage: it slows down diskette accesses and causes time-outs, which can make some diskette drives unusable.
I still cannot see how the Brain virus prevented copying.