Is there a virus that can kill a network of computers based on a simple router and switch in an hour?

Question

System administration

Is there a virus that can put down a network based on a simple router and switch in half a day? I serve a state organization and they use the network on the router and 30 clients that are not configured in any way! and don't be afraid! free avast and usb disk security on all computers!
Are there viruses that can put all 30 computers connected to the router through a simple switch? name if any? that is, so that they do not load and or are encrypted! all 30!
does avast know all of them?