The Duplicitous Demographers is the name that I regularly give to a small squad of plaguebearers that I field. What's in the name? Well, plaguebearers in the source materials (e.g. The Lost and the Damned) are well-noted for counting diseases, pestilences, virii, each other and nurglings amongst other things. I see this as a little bit like a census. One name given to a person whose job it is to study the statistics of people and a census is a demographer. The word duplicitous is both an alliteration conjoined with demographer, as well as indicating the fickle (evil?) nature of warp beings & daemons.
The models pictured here are a complete mixture of old Rogue Trader era plaguebearers, 1990's miniatures and the present day plaguebearers; all metal models. They blend remarkably well despite being removed from their molds over a time span of up to 20 years!
Gaming & Tactics.
On the tabletop, I use small squads of plaguebearers like the Duplicitous Demographers to hold objectives. This is their primary, and perhaps sole, duty to fulfil. As such, I don't bother with any upgrades such as noxious touch, or even a musical instrument. i.e. The model with the red plaguesword is simply a normal trooper as pictured here.
Given their purpose, the plan is simple. Run towards the nearest objective and go to ground! Since they're plaguebearers, they are remarkably resilient to incoming fire and will usually stick around for a good long while, hopefully the entire course of the game. If charged directly, they can usually hold their own until reinforcements arrive.
Typically, we can evaluate the worth a unit by comparing the points cost of the unit to the points cost of enemy units they take out of action. This pragmatic approach to evaluate worth is not the right mindset to use when thinking about objective holders. If the purpose of a unit is to hold objectives, then I argue that it is binary evaluation: either they succeed or they don't. In choosing an objective holding squad, plaguebearers are possibly the cheapest per model and most effective at this battlefield role in the game. Death Guard are slightly better due to their power armour, but they lack an invulnerable save and cost more points per model. Death Guard are more mobile and flexible as well and should have other primary roles, but they can certainly take up this role with excellent effect. Hence, I strongly believe that plaguebearers are the best troops choice in the game for the singular role of objective holding.
That really only leaves the question of how many to take in the squad for their objective holding task. It is fluffy to take 7 given that that is Nurgle's associated number. Seven is perhaps just a little bit on the low side though to hang on for the entire game in the face of strong incoming fire barrages (e.g. Tau), and hence by my previous argument is mildly likely to be uneconomic; especially given their hardiness is not that of a marines. I'd favour about 10. If going up to 14 (a multiple of 7 and still fluffy!), I think that I'd want a slightly modified battlefield role for the unit (first wave icon bearer?). In a pure Epidemius list, both smaller & larger units are beneficial and can take up, indeed must take up, mixed roles. I'll finish by noting that their mileage and role in earlier editions varied slightly, but I intend to leave that undiscussed presently.