<prompt> Echoviruses are cytopathogenic human viruses that mainly infect the:Respiratory systemCentral nervous systemBlood and lymphatic systemsIntestinal tract  <response>: Echoviruses are cytopathogenic human viruses that mainly infect the intestinal tract. Echoviruses were discovered accidentally during studies on poliomyelitis. They were named enteric cytopathogenic human orphan ECHO viruses because, at the time, they had not been linked to human disease and thus were considered orphans. Echoviruses now are known to infect the intestinal tract of humans; they also can cause aseptic meningitis, febrile illnesses, and the common cold. Echoviruses range in size from 24 to 30 nm in diameter and contain a core of RNA. 