The virus you've heard about is probably Mirai.
We've had a few questions recently that might be helpful to read for context, because your question covers several aspects of those:
In theory, if your router prevents all incoming connections, it makes it significantly harder for Mirai to get in and infect your devices. The publicly-available source code of Mirai seems to suggest that it just sends packets to as many IPs as it can, and if there's a reply, it then randomly tries the default passwords that it knows. I wrote an answer previously about that, if you're interested.
My concern would be that if Mirai did manage to enter your home network—through just one misconfigured or insecure device—it would render all of your firewalls and security useless. One of Mirai's IP ranges to check is
192.168.0.0/16 (your router's private network), so Mirai would almost certainly spread through all your vulnerable devices.
The solution to stop Mirai from being able to attack your network is simple—change every device's password from its default, and restart the device. If you do that, Mirai can't attack even if your device is accessible through the Internet (not to say it's a good idea to make things accessible if not necessary, though!).
There is a list of vulnerable devices here, or you can run Incapsula's scanner. Note that those will only check for vulnerability to Mirai—other viruses may operate differently, and following the suggestions in 'Securing small home automation setup' is probably your best bet.