if clamdscan returns an error then the file is marked infected.

[nss213]

In worker.rb, there is a line
if system("clamdscan #{fileName}")

In the case of an error, clamdscan returns status 2. Since the code doesn't examine the status, the file is treated as infected and (may be) deleted. This is probably the wrong thing and perhaps an error status should be reported on the topic?