The Phages are not used to cure viruses.
The Phages are bacteria that cause plagues.
Then, to cure some viruses or plagues, we use a weak version of the same virus or phage, to make our body react to it, while it is not that dangerous as the original one in full strenght.
So, if the UA is linked to the Plagues, it's because it is spreading it, not cureing.
My opinion btw.
Actually, phages are viruses that infect bacteria. As such, they can be used to (experimentally) fight off a bacterial infection. In and of themselves, bacteriophages are not harmfull to humans or other organisms outside bacteria.
Even then, viruses are very specific and will only infect one type of cell/animal. Zoonosis (where an infection jumps a species) are relatively rare.
A bacteriophage operates by injecting its DNA into a bacterium whose cell organelles will start to reproduce the proteins which make up the bacteriophage instead of those proteins it needs for its own survival. Basically, the bacterium will start to produce more phages.
No idea how this would be related to what we see, because if it is a container for a disease, why make the container look like a blown up version of that which it contains. And if it is supposed a single phage, you do not want to meet the bacterium it is supposed to infect. And that would mean the 'pods' (sorry, biscuit rolls) are what is actually supposed to match up to the outside of the bacterium and fit in there.
Anyone seen anything with three round holes in a triangular shape in the game?