Not using mIRC is not a virus-free guarantee. Only staying off the net is.

If you use mIRC only for chat and the occasional DCC between mates then there shouldn't be any problems. If you linger in wares channels or leave P2P programmes running 24/7 then of course the situation will be alot different. There is a perception that downloading via P2P provides for a reduced risk of getting infected with a virus. This is completely false.

In short - chatting will not get you infected. Downloading anything will by chance get you infected.