Wikipedia
AGI-Plan was a memory resident DOS file infector first isolated at the Agiplan software company in Germany. Because of CARO standards that dictate that viruses should not be named after companies, AGI-Plan's technical name is Month 4-6. This name also violates CARO standards, but a more minor rule involving syntax. AGI-Plan is related to the Zero Bug virus, as both it and AGI-Plan prepend 1,536 bytes to files they infect.
AGI-Plan is not initially damaging until several months after the initial infection, hence its name. After activation, AGI-Plan will begin to corrupt write operations, which results in slow, difficult-to-notice damage overtime.
AGI-Plan is notable for reappearing in South Africa in what appeared to be an intentional re-release several years after. AGI-Plan never succeeded in spreading significantly beyond the isolated incidents in Germany and South Africa.