From the harry potter wiki:

An adequately Imperiused being is placed under the caster's total control and may be directed to do anything the caster wishes, including crimes such as murder, political corruption, embezzlement, and so on. Also, whilst under the caster's control, the curse may also endow the victim with whatever skills that are required in order to complete the task at hand, such as increased strength or allowing them to cast spells far above their level. For example, an Imperiused Neville Longbottom was able to perform a series of "quite astonishing gymnastics" under the curse that he would not normally be capable of.[1]

I haven't read all of the original books, but is that bit about casting spells far above their level true? If so, it suggests a fairly easy way to gain control of an entire population: imperius a few wizards with the order to imperius as many other wizards as possible with this same command, and to await further instruction. If imperius curses have some sort of time limit after which they expire, include an additional order to seek out another imperius'd wizard to re-imperius you every x days.

This should spread exponentially like a particularly nasty virus.