At one time it was the most widespread of PC viral programs. In spite of this, and in spite of the existence of address and phone number information for the supposed author, we still have no first, second or even third hand reports of the production of the virus, and so little can be said with absolute certainty.
Brain is a boot sector infector, somewhat longer than some of the more recent BSIs.
As the boot sector is only a single sector, Brain, in infecting a disk, reserves two additional sectors on the disk for the remainder of itself, plus a third for the original boot sector. Brain is at once sly and brazen about its work. Hence the name of the virus. Well, it's quite simple really. In one of the most common Brain versions you will find text, unencrypted, giving the name, address and telephone number of Brain Computer Services in Pakistan.