Koi sleepy disease (KSD) of koi and farmed and wild carp receives considerable attention especially due to the economic significance of carp farming. Since the 1950s, the cultivation and breeding of carp species have been intensively incorporated into aquacultural food production and international fish trade. Thereby driving the spread of carp diseases to several localities. Nearly a half-century has passed since the 1974 discovery and isolation in Japan of KSD's causative agent, carp edema virus (CEV). Infections caused by CEV continue to emerge increasingly in carp populations around the globe with variation in disease occurrence respective to the host environment. CEV has been categorized in the family poxviridae and distinct genogroups of viral mutations are molecularly identified from virus isolates in different localities. Scientific knowledge about this virus, however, remains inadequate relative to the needs of researchers in aquatic health. The current review draws an attention to the nature of CEV as a non-classified poxvirus and its interaction with host and environment based upon the existing literature and the author's views. •CEV infections continue to emerge increasingly in carp populations around the globe•Multiple and distinct genogroups of viral mutations have been molecularly identified•Massive carp breeding and culture for international trade contribute to KSD spread•The knowledge gap in CEV understanding impedes serious progress for disease control•The theme of this review has potential prospective application in CEV study