Virus-infected systems do not operate normally when viruses occupy memory and disk space. Viruses can live at very low levels in the operating system and interfere with operations. Errors in viruses will cause unexplained crashes and strange behavior. Please take the virus problem seriously. Other types of carriers could be programs from Bulletin Boards or FTP sites that have been either tampered with or specifically designed as viruses but disguised as useful programs.
Viruses can live in other places besides applications.
They can also live in boot sectors on disks. The virus then hides in the system files and copies itself to the system RAM in order to reproduce and later, attack. In rare cases, an individual bent on harassing the user or sabotaging the computer system, modifies an existing program on that computer or copies a virus program onto someone's disk during their absence from their computer. It may also use a combination of these methods. The spread of viruses is like a domino effect.