During January 1986, the first computer virus was found in the wild, which could automatically spread from PC to PC.  Today, we encounter 20-30 new variants per day with innovation in their social engineering approach and their overall sophistication.  Users always need to employ the best technical defenses, stay up-to-date on all security patches, and "think before they click" any URL or email attachment.

PC viruses hit 20 year milestone 
http://news.bbc.co.uk/2/hi/technology/4630910.stm

It was during the opening weeks of 1986 that the first PC virus, called Brain, was discovered in the wild.   Though it achieved fame because it was the first of its type, the virus was not widespread as it could only travel by hitching a ride on floppy disks swapped between users. Brain was known as a "boot-sector" virus because of the area on a floppy disk it hid on. By concealing itself in this region, the virus could ensure that it would be installed every time that floppy disk was used on another computer.