How do viruses spread so quickly?
This is a very complex issue. Most viruses don’t spread very quickly. Those that do spread widely are able to do so for a variety of reasons. A large target population (i.e., millions of compatible computers) helps… A large virus population helps… Vendors whose quality assurance mechanisms rely on, for example, outdated scanners help… Users who gratuitously insert new software into their systems without making any attempt to test for viruses help… All of these things are factors.