Echovirus 9 (also known as E-9, E.C.H.O. 9, and formerly Coxsackie A23 or A23 virus) is a

serotype A serotype or serovar is a distinct variation within a species of bacteria or virus or among immune cells of different individuals. These microorganisms, viruses, or cells are classified together based on their surface antigens, allowing the epi ...

echovirus Echovirus is a polyphyletic group of viruses associated with enteric disease in humans. The name is derived from "enteric cytopathic human orphan virus". These viruses were originally not associated with disease, but many have since been identifie ...

. When first discovered, it was labelled as a

coxsackie A Coxsackie A virus (CAV) is a cytolytic Coxsackievirus of the ''Picornaviridae'' family, an enterovirus (a group containing the polioviruses, coxsackieviruses, and echoviruses). Viral structure and genome Coxsackie A virus is a subgroup of en ...

virus, A23. It was later discovered that A23 was an echovirus antigenically identical to the already-known echovirus 9. Echovirus 9 is the most common enterovirus type. It is a common cause of illness in humans, although unlike many enteroviruses, it rarely infects infants. Its transmission is facilitated by crowded conditions. Those who are slightly ill and children are at particular risk of contracting echovirus 9 (A23).

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