The Virulent Vectorium detachment rule says:
If you control an objective marker at the end of your Command phase and a DEATH GUARD unit from your army (excluding Battle-shocked units) is within range of that objective marker, that objective marker remains under your control until your opponent’s Level of Control over that objective marker is greater than yours at the end of a phase. In addition, until you lose control of that objective marker, it has the Nurgle’s Gift ability as if it were a DEATH GUARD model from your army.
Now in 10th the part about objectives gaining Nurgle's Gift wasn't too strong as objective markers were 40mm circles as by the time you were closes enough to be effected by this the enemy was likely in range or your own models or able to take the objective and turn it off.
In 11th this has changed now that objectives are terrain foot prints, often four 7" x 11.5" rectangles and either two 8" x 11.5" polygons or them combined together into a huge 10ish" x 11.5" rectangle right in the middle. That means that by the third round when Nurgle's Gift has a range of 9" you can have gigantic bubbles of it; 25" x 29.5" on the smaller ones and 28" x 29.5" on the big one.
Of course this is assuming you can control the objectives and that the rules work the same in 11th but from what I can see it will.