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Eastern virus to jump the Iron Curtain. When the virus was examined, researchers discovered the text string 'Vacsina', which immediately gave a name to Version 5.

Meanwhile, Teodor continued experimenting. By 15 December 1988 he had advanced to Version 8. On this variant the payload – the innocuous beep – now sounded only when an infected computer was restarted from the keyboard (a 'warm reboot'), allowing it to remain hidden for longer. In the best programming tradition, all his improvements were duly documented and given version numbers as they appeared.

Later in December a new Bulgarian virus was discovered. It carried a text string which said it had been authored by a Vladimir Botchev. The bug was almost certainly written in response to one of Vesko's magazine articles: in November Vesko had stated that it would be 'difficult' to write a virus that could infect all EXE files, including the longer ones, and Vladimir had presumably seen that as a challenge. His virus appeared less than a month after the article was published. It employed a novel and technically elegant device that enabled it to attach itself to any EXE file, no matter what length. After it infected a file it played the tune 'Yankee Doodle' – in celebration, perhaps.

This virus was generally not damaging – its payload was the tune – and because it was easy to detect, it never spread. But the new bug's payload was immediately copied by Teodor in his new variant, Version 18, which appeared on 6 January 1989. This one didn't beep; instead it played 'Yankee Doodle', which Teodor had lifted, note for note, straight from Vladimir's program.

Five days later, Teodor produced Version 21, which could remove the virus from infected files if a more recent version of this bug attacked the same system. Then, on 6 February 1989, Version 30 appeared. It incorporated a 'detection and repair' capability, that would warn the virus if it had been modified or corrupted while replicating. Eerily, it could then fix the damage itself by changing the corrupted instructions back to their original form. It was a kind of artificial life, though the repair capability was limited (it could handle only changes of up to 16 bytes in length).

By the end of February Teodor was on to Version 39 and his virus was now full of tricks: it could infect EXE files of any size, it could

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