Written after seeing episode 6
In the most recent episode they say that the hive mind people will starve to death in a few years. I believe this has been happening for millennia, where the virus reaches a new planet and then it takes over the planet, the intelligent life in that planet faces starvation and they decide that the only way to keep living is to beam their genetic code into space where other intelligent beings can pick it up and hopefully keep the hive mind alive.
Effectively, the virus with its intent to keep spreading, ends up causing large scale extinction of all intelligent life in the universe and that is why the universe is devoid of intelligent life.
Carol will realize this in the last or penultimate episode and points out the irony to them.
Their way of life, to not hurt any other living thing is actually causing large scale extinction on a galactic scale.
The hive mind realizes this decides that the most logical way forward that causes the least harm to life in the long run is to end their existence.
They derive a cure using the antibodies that the immune survivors have developed against the virus and decide to end themselves.