Benign viruses replicate, but do not do anything malicious. For example, they may beep or display messages on the screen, but they do not do intentional damage.
Malignant viruses do attempt to damage computer resources such as erasing a hard drive.
Viruses can be dormant for days or months before becoming activated. Hackers intent on malicious destruction of networked systems to which they have gained unauthorized access can also introduce viruses to shared systems. The risk of getting a computer virus cannot be completely eliminated. Virus detection software should be used. There is a variety of virus detection software available.