New viruses and new strains of existing viruses show up every year. One recently-infected business really believed itself protected. After all, they had purchased a virus protection program over three years ago and hadn't had any problems since. It was my sad duty to inform them that approximately 1,000 new viruses had since been released. Some virus creators have released their destruction via their own shareware programs.
Virus programs can lie dormant for days, weeks, months, and even years.
They can sit and wait for a specific date or time, or for a certain chain of events to occur, before wreaking havoc on your system and its files. In most cases, a virus will take every opportunity to replicate itself, often without any outward sign. In general, the most common way for a virus to enter a computer is through the use of pirated software or shareware from an unknown source. These are typically handed down by unscrupulous users who consider the risks to be acceptable. Within 24hours, several viruses had spread from the used equipment to numerous other computers and a multitude of floppy disks.