Human civilization would not survive such a virus.
W.r.t. your questions about radiation, I believe you're referring to radiation sterilisation. We could make 'rad guns' if we put our minds to it, and in fact we have portable UV light sterilisation devices already. However, they're way way too small-scale to deal with the kind of outbreak you speculate. If the virus (or its microbial hosts) are so dense as to be seen in the air, they'll likely be a thousand times more plentiful in the ocean, ranging from the photic levels to the hadopelagic depths in a variety of host forms. Humanity would literally never be able to sterilise that volume of water, even if we turned our entire economic output globally to the production of radioactive materials to disperse, or submersible drones equipped with UV sterilisers. Cut off from the ocean, the economy would nosedive worldwide, as nations struggle to make up for the seafood shortfall, and trade dwindles to what can be carried by air freight.
That is of course, assuming the virus is so friendly as to stay in the ocean. Nearly all of the world's major cities are located at the coast, and those that aren't are along major rivers. If virus hosts get into the sewer systems and aquifers, there's no getting rid of them save ridiculous numbers of nuclear strikes to penetrate the bedrock, so the cities are lost, along with a large part of human technological knowledge and expertise.
Sure, we could hide underground. Deep underground, in solid shells of concrete with a nuclear reactor to keep us warm and lighted farms to convert that electricity to a more palatable form, perhaps emerging when our descendants have forgotten a world without a roof overhead. Except that this assumes we'll outlast and wait for the virus to 'starve out'. If the virus is intelligent and sapient, as it would be having assimilated so much of humanity, it could ration out the remaining biosphere, only consuming the bare minimum to stay (alive?) and letting the rest regrow. Or if it can incorporate photosynthetic tissues, then it would replace the biosphere entirely. More likely, it'll take an active role, secreting organic acids to melt through the concrete, within as little as years. The day the vault-dwellers hear the dripping of groundwater through their walls, they'll pass around the last bullets and turn out the lights.
Our best hope for survival would be the theatre which no form of Earthly life has yet to venture into save humanity - the absolute void of outer space. Entire nations would turn their dying days to building rockets to send their best and brightest into the void, while massive riots rage over who gets to escape the nightmare. Much would depend on the rapidity of the outbreak - would we have time to build and hurl aloft hydroponic farms, knowing that the survivors would never be able to trust any resupply from home? The primitive colonies would have to stay separated - can't afford to keep all your eggs in one basket, after all. Except that limits the variety of eggs available - humanity would face a genetic bottleneck unlike any in our history, with the number of viable couples numbering only in the double digits. Assuming that the survivors, knowing that everyone they ever knew is dead or dying, confined to a bubble the size of an apartment with the same people for the rest of their lifespan, did not go insane and vent the airlock.
Perhaps some lucky few would make it to the moon or even Mars, dispatching jerry-rigged capsules down the gravity well and digging homes in the lunar regolith while staking out solar-panelled lawns. Against all odds, they might re-establish some semblance of civilisation if they had the numbers, bootstrapping themselves up from what they brought with them, scavenging ice from comet craters and manufacturing electronics from lunar silicates. It'll be a faint shadow of what our civilisation was capable of at our peak, but they'll be human and surviving, if not thriving.
Until decades later, when they catch sight of the bright flares of chemical rockets rising up from the Plague World like grasping fingers. Who knows how they'll react?
Dang, I'm getting the urge to write a novel about this now.