Signum, also known as BPL is a virus from 1987. It is the first virus for the Atari ST computer. At one point the virus's population was over one million.
Signum becomes memory resident after booting from an infected disk. It can infect any drive on the A: or B: drives.
This virus is at one point said to have spawned 1.5 million copies.
Richard Karsmakers. UVK2000 Support Page, THE KNOWN VIRUSES ON ATARI TOS COMPUTERS AND THEIR SYMPTOMS.
Hallvard Tangeraas. Common viri [sic on the Atari ST]. 1996.02.15