Nobody can reliably say which computer virus is most common today, because modern worms are often designed to spread without being noticed, according to security expert Bruce Schneier. For example, estimates of the spread of the Storm Worm released in 2007 ranged from 1 million to 50 million computers.
It is possible to estimate how common a virus is by measuring how quickly it spreads, even though modern viruses make this difficult to determine. Antivirus companies sometimes release their best estimates. One of the most historically widespread and devastating viruses was the 2004 MyDoom virus with over 2 million infections, claims Norton. The 2005 Samy XSS worm that struck MySpace is known to have infected over 1 million computers within 20 hours of release.