At this point there are four viruses that are of particular concern during the Y2K transition. The first three of the viruses are Microsoft Word macro viruses. The last virus, PC CIH, is an older, more common virus, but still very prevalent and has a very destructive payload and could cause very severe damage to infected machines. All variants of Marker steal some type of information from infected machines and then covertly FTP the information to a specified IP address. Class is polymorphic, which means it is capable of mutating its signature, thus making detection and removal more difficult.
Class also exhibits stealth characteristics, which further complicates detection and removal.
In current versions the payload is not destructive. Melissa spreads as an email message with an infected Microsoft Word document as an attachment. Despite the age of the original Melissa virus, the numerous variants of Melissa and their ability to spread constitute a continued threat. PC CIH was first seen in the summer of 1998, but is still spreading very rapidly. PC CIH has a very destructive payload which will delete the first megabyte of data on all hard drives available to the infected machine and overwrite flash BIOS memory.