You can't just be infected by viruses by joining channels. You have to accept files and then execute them for that kind of thing to happen. This is all assuming you're using the latest version of mIRC (7.17), of course, and have other security programs in place to protect you if you need it.
You can ignore these people. They are likely doing it to every user who joins in order to scare them. It looks like they've succeeded with you.
