The retrovirus contains fragments of these anceint entities. The PCs are all infected with pieces of the same being. It doesnt think like us. They do not have a collective consciousness or anything, but they feel like they should work together. At first they don't know why, or come up with thier own rationalizations, and later when they start asking "why am I doing this" they find they simply can't leave. They get on each other's nerves, they develop bonds and conflicts, but are compeled to stay in touch with each other, to work togther to occasionally do inexplicable things. They are addicted to one another's presence, or at the very least communication. Addicted in the sense that they have developed a biochemical dependence on each other. The part of them that is the ancient entity resonates with the pieces of itself in the others, making them sick or confused if they part company for long periods of time. Increasingly shorter periods of time.
The entity has some ancient grudges, instictual likes and dislikes, that puts the PCs in conflict with other similarly infected groups. At some point they should find a fully intigrated group, one that does think and act with a single mind. They come into contact with others that they feel they should like, or get along with, but for some reason keep rubbing each other the wrong way. Some people that they don't like, or wouldn't, they feel compleled to help.
Explain to the players that there is a story reason for them to work together, or at least feel like they should, that will occasionally involve you telling them how they react to certain things, perhaps allowing some kind of Will save or similar mechanism if they try to fight it or go against the group or their instincts. React to the player's natural interaction to perhaps explain sub functions of the entitiy, its need to belong/to fight/to think rationally/etc.
Now, this may get a little horrific and may be an altogether different direction that you meant to go, but I think it would be very intersting to see it play out, and fun for the players to find out whats happening to them as it goes along.