The "sluts" comment was an appearance I was hoping to avoid (i.e. if the disease is passed through sex, then only those sexually promiscuous would be infected). By keeping the disease viral, anyone could be infected, regardless of their background.
Decided this needed a minor augment. Just because it's "viral" doesn't mean its wildly contagious. Different virus strains spread by different means. Such as HIV/AIDS for example, it is primarily a STD, but can also be picked up by way of exchanging bodily fluids(specifically blood, others bodily fluids are technically possible, but the quantities involved are so large, its unlikely).
Of course, in that context, Ebola qualifies as a STD, among other things. :)
But even with a blood borne illness, as happened with HIV/AIDS initially, they viewed as a standard STD at first, and had no reliable means of detecting it (at contagious levels) once they did bother to actually look for it in blood banks for some time. So that is a viable contagion vector.
Which is where my earlier comment about how contagious it is vs how quickly symptoms start to show + how quickly the symptoms are (Correctly) identified as being viral and the means of transmission determined.
If it plays out slowly enough you get the scenario where practically anyone who needs regular blood transfusions + anyone who has sex outside of a (truly) closed group of people ends up infected before anyone realizes there is a problem. But that also means the buildup would have to be very slow, and practically inevitable for most people once it's realized that its a thing(because they already have it, and are presumably contagious well before their symptoms become more obvious).
The only question is if there are ways to "Accelerate" the progression of the illness at that point(but that also creates problems with Patient 0 and Patient 1 to account for how they didn't inadvertently cause each other's symptoms to develop more quickly until reaching the point of being "a noticed problem")
Of course, there are other ways to play with it too. If it's a blood based illness, there is nothing saying there couldn't have been some kind of conspiracy to deliberately infect people with it using some kind of vaccine ploy to either initiate the "patient 0 phase" or to "kick over" the zombie plague from operating in a "Carrier" mode to a "zombie" mode, where the "kick over" agent may very well be airborne and highly contagious. So then you'll have people with the primary infection, and those with the secondary infection. Possibly even people with the secondary infection without the primary infection(which could be interesting in other ways), but as long as a person doesn't have both, they're safe from becoming zombies.
There are other ways to do it too, something in the drinking water, aerosol spray of some type (lawl Contrails) and so forth.