The first virus specifically written for Microsoft Windows 98 is shipping and available before the operating system itself: Windows 98 ships June 25.
HPS (Hantavirus Pulmonary Syndrome) is a 32-bit polymorphic Windows virus that only activates when the infected system is booted on a Saturday.
The Hantavirus Pulmonary Syndrome for which the virus is named is a biological disease transmitted by rodents. Its principal symptom is an acute respiratory crisis.
If activated, the computer virus flips any uncompressed bitmaps horizontally, only on Saturdays. This produces a "mirror" effect for many of the screens used within the Windows operating system, according to Virus Bulletin, the Oxfordshire, England-based technical journal that tracks viruses.
The virus was authored by GriYo, who is referred to as a "notorious member" of the 29A virus-writing group, and is credited with writing the complex Implant virus, according to Virus Bulletin.
GriYo did make the virus backward-compatible, though, so Windows 95 users need not upgrade to the new OS to experience the same effects.