Cross-Platform Viruses
Besides security differences, Windows and Unix have fundamentally different designs. They use different methods to store files, access the computer's processor in different ways, and organize their applications and memory with different schemata. Therefore, a virus that attacks a Unix system will not necessarily be able to attack a Windows system. An exception might be if an application that runs on Unix and Windows has a known virus. However, this would just make it an application-specific virus that virus scanners could pick up.