Hi Sean!
Answering a message from Sean Turner to John Kismul:
 JK>>> What is the difference between a virus and worm?
 JK>> So the worm will execute itself many times and make a mess.
 ST> if it replicates and doesn't cause any other problems it's a worm? 
No.
 ST> yet if it causes some sort of damage it's a virus? 
No.
 ST> This has always been unclear to me. And this isn't directed only to 
hn.
Let's have another try.
A virus is a sort of program which has to use another program to be 
*started*. Once it has been started and is active in memory, very often like 
a TSR, the virus is able to watch all the operations on the computer and to 
replicate itself (by copying itself at the end of another program and 
changing the entry point of that program in its header. That's the way 
file-infectors act).
The virus also can have a 'payload', any sort of routine able to delete 
files, directories or just slighly change a few, randomly choosed, sectors of 
files (data or program) to be written on the harddisk (like RIPPER does).
BUT: not every virus is containing such a 'payload'. Some of them are 'badly' 
implemented (luckily for us) - they never come to action. And some of those 
'payloads' doesn't do harmful things.
A worm is a program on its *own*. It has to be started by a user or a special 
system-event by calling it. Once started it's trying to get as much CPU-time 
as possible. Easiest way to do this is to replicate itself more and more thus 
occupying the systems memory and CPU-time. Worms are especially deadly for 
multitasking/multiuser systems like UNIX and, as far as I know, they exist 
only in variants written for this operating systems.
Both are able to cause different sort of damage if their programmers 
implemented such sort of routines. The difference is the way they come to 
action.
A virus always by calling *another* program from which you do not know that a 
virus is at its end and first executed, trying to infect as many other 
programs as possible.
A worm by calling the program itself - you maybe do not know that this prgram 
actually behave like a worm though. Then it replicates itself but *does not* 
infect another program.
Greetings from Wiesmoor (Germany)
Klaus
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