There are basically two types of viruses, innocuous and malicious. Innocuous viruses may beep or display messages, but do not intentionally damage software or data. It is important to realize that innocuous viruses can damage your system even through the damage is unintentional. Malicious viruses, in addition to replicating, attempt to damage your hard drive or floppy disk. 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.