The PC was typically infected by booting from an infected diskette. Computers, at the time, would default to booting from the A: diskette drive if it had a diskette. The virus was spread when a floppy diskette was accessed with an infected computer. That diskette was now, itself, a source for further spread of the virus. This was much like a recessive gene - difficult to eliminate - because a user could have any number of infected diskettes and yet not have their systems infected with the virus unless they inadvertently boot from an infected diskette. Cleaning the computer without cleaning all diskettes left the user susceptible to a repeat infection. The method also furthered the spread of the virus in that borrowed diskettes, if placed into the system, were now able to carry the virus to a new host.