McAfee, a Silicon Valley security firm, studying all the types of modern cyberthreats, states that China at the moment has the largest number of compromised ("hacked") computers. The estimated number provided is 1,095,000 computers in China (and 1,057,000 in the USA).
The computers are often work in so called botnets; a botnet is controlled from a hidden command center and can perform a number of actions. Among those are: sending spam (a compromised, 'zombie" computer may send hundreds of thousand spam messages daily), attacking other computers (i.e, performing DDoS attacks) and so on.
Kraken, a notorious botnet of 2008, united more than 400,000 infected computers. The malware that performed the infection and joined the computer to the botnet was capable of self-modification and evaded most of known antivirus and antimalware tools.
Infected computer may perform the malicious actions in a manner that can't be easily detected. The computers hacked are not all home computers; Kraken managed to infiltrate many corporate networks, passing by firewalls and other filters undetected.
There are hundreds of botnets in cyberspace. No amount of thorough monitoring and security precautions is enough to reduce the cyber-zombie army significantly: there are many factors that donate to the botnets growth.
First, installing the proper software to make a computer guarded against any type of malware requires certain level of knowledge and education.
Second, such products, able to address "zero day attacks" as well, aren't too cheap.
Third, a significant amount of unlicensed OS installations are vulnerable only because they aren't allowed to install security updates.
Botnets are often controlled by criminals. Since large botnets can do qutie a lot of damage (they can "shut down" virtually any site or other network service), their existence is a direct threat to any country; and the more country depends on Internet, the more vulnerable it becomes in terms of cyber-threats.
You may learn more from a Damballa, a company dealing with botnet threats. But to be able to withstand the threat, one simple condition must be net: all the computer users should be literate enough to guard their computers and to prevent donating to someone else' grief.
Botnet controlling scripts can be easily found and downloaded free of charge. This very fact should be impressive enough to think of personal computer security as of a conditio sine qua non.
