The FILE Virus is a simple UNIX virus.  It was written by the author Silvio
Cesare.  It uses File infection for replication.  To understand this method,
imagine the following..

# cat host >> parasite
# ./parasite

In this example, although functionality of the host has been lost, its still
intact within the new parasite.  If the parasite knows its own size, it can
manually extract the host from the file and execute it.  The FILE Virus does
this but before executing the original host, it forks, putting the exec
function in the child process and waiting in the parent.  It does this so it
can clean up the temp file used for extracting the host.  Direct infection
for propogating is used.  It does this by infecting executeables in the current
directory.  This virus is not strip safe; stripping an infected program will
strip the original host leaving only the virus and making for an unrecoverable
infection.  To avoid reinfecting the same host, a "magic" number is appended to
the end of the file, this number is then checked in the infection routine.