(..where by 'undetectable' I mean, 'undetectable as a virus' -- the technique is to bypass AV scans by distributing modules of code that are themselves innocent

(..where by 'undetectable' I mean, 'undetectable as a virus' -- the technique is to bypass AV scans by distributing modules of code that are themselves innocent but combine in dangerous ways; the interview I read with one of the guys at Kaspersky described it in detail.)