A computer virus modifies other legitimate host files in such a way that when a victim’s file is executed, the virus is also executed. Pure computer viruses are uncommon today, comprising less than 10% of all malware. Worms have been around even longer than computer viruses, all the way back to mainframe days. Trojans are hard to defend against for two reasons: They’re easy to write (cyber criminals routinely produce and hawk Trojan-building kits) and spread by tricking end-users.”]
Source: https://www.csoonline.com/article/2615925/security-your-quick-guide-to-malware-types.html