viruses are malware for a network..!
rootkits, trozan horse are the common dangerous one.
actually, virus are created by computer programmer , to be concise the cunning and fraud ones..with the motive of cyber theft or other cyber crimes.
Anti virus are developed to counter these viruses.
virus are not made for the upliftment and goodwill of computer network and community rather for evil deeds .
Hope it helped a bit ;-)