In the six or seven years that we have been publishing the Microsoft Security Intelligence Report (SIR) I have seen many trends emerge over time.
The threat landscape is constantly changing as attackers try to find methods that will help them compromise the systems they target. For several years viruses (file infectors) seemed to be out of favor with attackers as they used other categories of threats to attack systems.
Viruses simply didn’t support the profit motive many attackers had in the same way that Trojan Downloaders and Droppers, Miscellaneous Trojans, and Password Stealers and Monitoring Tools all did. Viruses are threats designed in an era before ubiquitous Internet connectivity made it easier for Worms to successfully self-propagate. Worms like SQL Slammer and Blaster spread around the world in minutes. This would likely take an old fashioned file-infector much, much longer to accomplish, limiting their ability to infect large numbers of systems quickly. Additionally, Viruses tend to be relatively “noisy” threats as they typically try to infect large numbers of files (.exe, .dll, .scr) on the systems they compromise. This characteristic can make them easier to detect than other more blended threats.