Latest estimates reported in the press suggest that more than 2.2 million PCs based in the USA were hijacked by cybercriminals in the first half of 2010, and used as part of a botnet.
And what's one of the principal reasons why cybercriminals compromise innocent users' PCs? To send spam without their knowledge.
It's still a surprise to many people who don't work in the field of computer security, but the vast majority of the spam you receive in your inbox is not sent from the spammers' own computers but relayed through infected PCs belonging to regular members of the public.
In other words, Aunty Hilda could be the one pumping out all that Viagra spam.
And where are most of the world's Aunty Hildas? The United States.
Here are the latest stats from SophosLabs, showing the "dirty dozen" spam-relaying countries for the third quarter of this year.
The top twelve spam relaying countries for July - September 2010
|7= S Korea||3.0%|
Top spam-relaying continents, July - September 2010
|3. N America||22.3%|
|4. S America||11.5%|
You should never even be tempted to open a spam message out of curiosity, as it can only take a second to effectively hand over control of your computer to the spammers. If your computer does become part of a botnet, you're also inviting further malware infections, which may compromise your personal or banking details.
The best way for users and administrators to reduce the risk of being compromised is to run anti-spam and anti-malware protection, behave sensibly when online, and ensure systems are up-to-date with security patches.