There are more MS-DOS/Windows viruses than all other types of viruses combined (by a large margin). Estimates of exactly how many there are vary widely and the number is constantly growing.

In 1990, estimates ranged from 200 to 500; then in 1991 estimates ranged from 600 to 1,000 different viruses. In late 1992, estimates were ranging from 1,000 to 2,300 viruses. In mid-1994, the numbers vary from 4,500 to over 7,500 viruses. In 1996 the number climbed over 10,000. 1998 saw 20,000 and 2000 topped 50,000. It's easy to say there are more now. Indeed, in April 2008, the BBC reported that Symantec now claims "that the security firm's anti-virus programs detect to 1,122,311" viruses and that "almost two thirds of all malicious code threats currently detected were created during 2007."

The confusion exists partly because it's difficult to agree on how to count viruses. New viruses frequently arise from someone taking an existing virus that does something like put a message out on your screen saying: "Your PC is now stoned" and changing it to say something like "Donald Duck is a lie!". Is this a new virus? Most experts say yes. But, this is a trivial change that can be done in less than two minutes resulting in yet another "new" virus.

More confusion arises with some companies counting viruses+worms+Trojans as a unit and some not.

Another problem comes from viruses that try to conceal themselves from scanners by mutating. In other words, every time the virus infects another file, it will try to use a different version of itself. These viruses are known as polymorphic viruses.

One example, the Whale (an early, huge, clumsy 10,000 byte virus), creates 33 different versions of itself when it infects files. At least one vendor counted this as 33 different viruses on their list. Many of the large number of viruses known to exist have not been detected in the wild but probably exist only in someone's virus collection.

David M. Chess of IBM's High Integrity Computing Laboratory reported in the November 1991 Virus Bulletin that "about 30 different viruses and variants account for nearly all of the actual infections that we see in day-to-day operation." In late 2007, about 580 different viruses, worms, and Trojans (and some of these are members of a single family) account for all the virus-related malware that actually spread in the wild. To keep track visit the Wildlist, a list which reports virus sightings.

How can there be so few viruses active when some experts report such high numbers? This is probably because most viruses are poorly written and cannot spread at all or cannot spread without betraying their presence. Although the actual number of viruses will probably continue to be hotly debated, what is clear is that the total number of viruses is increasing, although the active viruses not quite as rapidly as the numbers might suggest.

Summary

• By number, there are well over 100,000 known computer viruses.
• Only a small percentage of this total number account for those viruses found in the wild, however. Most exist only in collections.

Introduction to Viruses
Virus Behavior	Virus Names