For that to work, they would need the co-operation of the anti-virus companies out there. After all these types of viruses/worms are hardly new are they? Even if they somehow managed to persuade the anti-virus companies (which is highly unlikely), then I'm sure it wouldn't be long before details were posted on various sites.
Perhaps they assume that there are people who know about email encryption, but not about the dangers of running email attachments without scanning them first.
As an aside, the only viruses I have ever received were from friends, who did not realise they had (a) got the virus, or (b) were transmitting it