Maybe Brain was not the most destructive virus, but in 1986 it became the first  one , the target of which were the  personal computers running Microsoft's DOS operating system.

The virus occupied a huge part of the RAM and caused infection messages on the monitor screens

The virus also informed that it is necessary to call the provided phone number to deactivate the virus. The phone number belonged to two Palestinian brothers who developed the Brain virus. They assured that they did not want to create such problems, but only created a means of protection against copying for medical software, but there was someone who copied part of the software code. And the brothers received more than they had planned, worldwide requests for disinfection of the virus.

The moral side of history․ Be careful planning something