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I've not actually tried this myself (so proceed at your own risk) but here's a possible solution: Go into your CK/db/events folder and open the global_disease_events file in a text editor (notepad is fine).

You can see there all of the disease events listed like so:

Code:
#################################
# The Bubonic Plague on retreat	#
#################################
province_event = {
	id = 1071

	picture = "event_plague"

	trigger = {
		condition = {
			type = has_province_effect
			value = { bubonic_plague = yes }
		}
	}

	mean_time_to_happen = {
		months = 48
	}

	action_a = {
		effect = { type = add_province_effect value = bubonic_plague_immunity }
		effect = { type = remove_province_effect value = bubonic_plague }
	}
}

If you edit the mean_time_to_happen value to something lower (months = 24, for example) it should make the event fire quicker.
Make a backup copy of your global_disease_events file before editing it, just in case something goes wrong.

Another option would be to open your console in game (F12 key) and fire the event manually with the event ID.
 
If it lasts for 30 years in one (or several) province, it's bad luck, if not a bug.
On the other hand, plague can take decades to go through all of Europe, so if you have a large empire with territories in the Middle-East and in Spain or Britain, it's not that surprising if some of your provinces get hit 30 years after the first ones.
 
if it was the occasional province i wouldnt be bothered but over half my realm is still affected and the realm is compact rather than spread out. ive just played another 5 years and plague is still dying out very,very slowly.:eek: