Friday 13th is a well known computer virus. It is also called Jerusalem. This virus is famous for being very bad. It was first found in 1987. It was made to start causing trouble on Fridays that're on the 13th day of the month. Friday 13th was meant to hurt computers that use MS-DOS systems. It would delete files that the computer needs to work. Friday 13th became very infamous in 1988 and 1989. It caused a lot of problems, for people. That is why people started calling it the Friday the 13th virus.