Viruses are only excluded as they aren’t alive. So how would you play as a virus? Just by waiting because viruses cannot make any active actions. So that would mean just the Thrive editor would be used for gameplay and other than that you’d just wait. And in terms of hazards, viruses could be added but no one has volunteered to program them. A part is probably that such a hazard would be only a slight variant of a toxin cloud (one that does damage over time).
There’s been a bunch of threads about viruses already: