Antivirus?

[Grant]

Hi,

I was just wondering this morning, do i really need antivirus? the 
reason i thought that was because i run only linux and freebsd boxes and 
i heard that there is very limited virus's that can affect/infect a 
bsd/linux box...

So i was wondering do i really need to install them virus filters on my 
mail server? or on any of my bsd/linux boxes, if i do could someone 
explain why....

Thanks.
Grant.