Trend for viruses today is very much to keep a low profile while on an infected system. As a result we rarely see the extremely destructive viruses that cropped up in yesteryear. Modern day virus authors see a swathe of files left in varying degrees of corruptedness as a perfectly acceptable and possibly desired, side effect of a successfully infected system. Some members of the Virut/Vetor family will randomly choose not to leave an infection marker after infection. This leaves the way open to multiple infections (more headaches for anti-virus companies) but also increases the chances that the end file will be corrupt.”]
Source: https://nakedsecurity.sophos.com/2008/05/23/to-junk-or-not-to-junk/