（2）继发性：发病率约占80%，常见病因有：1）感染：可由细菌、病毒、支原体或疫苗接种等引起，病原体包括伤寒、链球菌、金黄色葡萄球菌、结核、肝炎病毒、巨细胞包涵体病毒、EB病毒、疱疹病毒、流感病毒、腺病毒、腮腺炎病毒及肺炎支原体等。
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