Brain is the industry standard name for a computer virus that was released in its first form in January 1986 and is considered to be the first computer virus for MS-DOS. It infects the boot sector of storage media formatted with the DOS File Allocation Table (FAT) file system.
The virus “Brain” was written by two Pakistani brothers, Basit Farooq Alvi and Amjad Farooq Alvi, 1986 who have over the years said the Brain was not a virus but a mechanism for them to protect their medical software from piracy. he virus was “not made to destroy any data”. Rather, it was intended to ensure that users whose machines had become infected due to the use of pirated software could contact them for “vaccination” (to buy original from them). Basit and Amjad never thought of the virus growing into a global-sized monster, with powers beyond their capacities to control it.

Birth of Anti-Virus

This opened the floodgates for newer variants and copycats which applied the same logic as Amjad’s to infiltrate computers and cause widespread damage. It was the Brain-computer virus that gave the idea to some programmers to write the first anti-virus software. John McAfee, a software engineer, and US millionaire is considered a guru of the anti-virus industry. 
A boot sector virus is a type of virus that infects the boot sector of floppy disks or the Master Boot Record (MBR) of hard disks (some infect the boot sector of the hard disk instead of the MBR).  Boot sector viruses infect at a BIOS level, they use DOS commands to spread to other floppy disks.
Since the MBR executes every time a computer is started, a boot sector virus is extremely dangerous. Once the boot code on the drive is infected, the virus will be loaded into memory on every startup. From memory, the boot virus can spread to every disk that the system reads. Boot sector viruses are typically very difficult to remove, as most antivirus programs cannot clean the MBR while Windows is running. In most cases, it takes bootable antivirus disks such as a Symantec/Norton AntiVirus (SAV/NAV) rescue set to properly remove a boot sector virus.

How Boot Sector Virus infect?

  • Shift or overwrite the original boot sector of a disk
  • Replace the boot sector with the virus itself
  • Generate bad disk sectors

Symptoms of the Boot sector virus

A boot sector virus can cause a variety of boot or data retrieval problems. In some cases, data disappear from entire partitions. In other cases, the computer suddenly becomes unstable. Often the infected computer fails to start-up or to find the hard drive. Also, error messages such as “Invalid system disk” may become prevalent.
How Boot sector virus spread
Any disk can cause infection if it is in the drive when the computer boots up or shuts down. The virus can also be spread across networks from file downloads and from email file attachments. In most cases, all write-enabled floppies used on an infected PC will themselves pick up the boot sector virus. 

Present-day operating systems include boot-sector safeguards that make it difficult for boot sector viruses to infect them. However, the best protection against boot sector viruses is a good antivirus program with up-to-date virus definitions.