It is, like all plagues it disappears when the plague is cured, the event simply forces a plague to happen in every settlement at once.

You can add an extra plague event like so however:

Code:
event	plague 		black_death_hits
date	266
position	281, 163
position	258, 134
position	233, 117
position	278, 109
movie	event/black_death.bik

event	plague		black_death_1
date 	268
position	278, 101
position	258, 104
position	213, 92
position	216, 88
position	209, 73
position	230, 38
position	232, 76
position	150, 70
position	132, 76
position	131, 92
position	120, 99
position	133, 104
position	146, 108
position	136, 111
position	157, 82
position	168, 95
position	190, 85
position	191, 71
position	175, 87
position	194, 99
position	93, 95
position	77, 73
position	92, 83
position	97, 114
position	105, 104
position	121, 122
position	129, 119
position	160, 108
position	249, 44
position	246, 76
position	241, 65
position	232, 76
position	245, 85
position	251, 57
position	254, 51
position	214, 65
position	202, 57

event	plague		black_death_2
date 	270
position	278, 101
position	258, 104
position	213, 92
position	216, 88
position	209, 73
position	230, 38
position	232, 76
position	150, 70
position	132, 76
position	131, 92
position	120, 99
position	133, 104
position	146, 108
position	136, 111
position	157, 82
position	168, 95
position	190, 85
position	191, 71
position	175, 87
position	194, 99
position	93, 95
position	77, 73
position	92, 83
position	97, 114
position	105, 104
position	121, 122
position	129, 119
position	160, 108
position	249, 44
position	246, 76
position	241, 65
position	232, 76
position	245, 85
position	251, 57
position	254, 51
position	214, 65
position	202, 57

event	plague		black_death_3
date 	272
position	278, 101
position	258, 104
position	213, 92
position	216, 88
position	209, 73
position	230, 38
position	232, 76
position	150, 70
position	132, 76
position	131, 92
position	120, 99
position	133, 104
position	146, 108
position	136, 111
position	157, 82
position	168, 95
position	190, 85
position	191, 71
position	175, 87
position	194, 99
position	93, 95
position	77, 73
position	92, 83
position	97, 114
position	105, 104
position	121, 122
position	129, 119
position	160, 108
position	249, 44
position	246, 76
position	241, 65
position	232, 76
position	245, 85
position	251, 57
position	254, 51
position	214, 65
position	202, 57

event	plague		black_death_4
date 	274
position	278, 101
position	258, 104
position	213, 92
position	216, 88
position	209, 73
position	230, 38
position	232, 76
position	150, 70
position	132, 76
position	131, 92
position	120, 99
position	133, 104
position	146, 108
position	136, 111
position	157, 82
position	168, 95
position	190, 85
position	191, 71
position	175, 87
position	194, 99
position	93, 95
position	77, 73
position	92, 83
position	97, 114
position	105, 104
position	121, 122
position	129, 119
position	160, 108
position	249, 44
position	246, 76
position	241, 65
position	232, 76
position	245, 85
position	251, 57
position	254, 51
position	214, 65
position	202, 57

event	plague		black_death_5
date 	276
position	278, 101
position	258, 104
position	213, 92
position	216, 88
position	209, 73
position	230, 38
position	232, 76
position	150, 70
position	132, 76
position	131, 92
position	120, 99
position	133, 104
position	146, 108
position	136, 111
position	157, 82
position	168, 95
position	190, 85
position	191, 71
position	175, 87
position	194, 99
position	93, 95
position	77, 73
position	92, 83
position	97, 114
position	105, 104
position	121, 122
position	129, 119
position	160, 108
position	249, 44
position	246, 76
position	241, 65
position	232, 76
position	245, 85
position	251, 57
position	254, 51
position	214, 65
position	202, 57

event	plague		black_death_6
date 	278
position	278, 101
position	258, 104
position	213, 92
position	216, 88
position	209, 73
position	230, 38
position	232, 76
position	150, 70
position	132, 76
position	131, 92
position	120, 99
position	133, 104
position	146, 108
position	136, 111
position	157, 82
position	168, 95
position	190, 85
position	191, 71
position	175, 87
position	194, 99
position	93, 95
position	77, 73
position	92, 83
position	97, 114
position	105, 104
position	121, 122
position	129, 119
position	160, 108
position	249, 44
position	246, 76
position	241, 65
position	232, 76
position	245, 85
position	251, 57
position	254, 51
position	214, 65
position	202, 57

event	plague 		black_death_7
date	280
position	205, 125
position	196, 114
position	99, 181
position	102, 74
position	90, 162
position	142, 187
position	106, 164
position	157, 136
position	172, 122
position	148, 147
position	141, 154
position	141, 166
position	174, 136
position	192, 140
position	163, 143
position	228, 2
position	268, 13

event	plague 		black_death_8
date	282
position	235, 169
position	202, 179
position	181, 174
position	188, 186
position	167, 183
position	169, 151
position	217, 139
position	212, 162
position	189, 158
position	155, 155
position	244, 158

This Code would need to replace the existing plague code however.