abc's dr. richard besser has the story. >> reporter: dr. angela lumba on onn of the worst e.r. days she's had. >> make sure he is okay. i was scared. >> reporter: frantic parents. wheezing children. >> the virus is called enterovir enterovirus. >> reporter: in the nine hours at st. louis children's hospital, we watched the waiting room fill up. penelope, her dad rushing her in. how old are? >> 3 1/2. >> reporter: one thing you should know about the virus -- it can come on quickly. >> noticed the fever today. and coughing. she got it from last night. >> reporter: at least 15 states grappling with suspected cases of the rare respiratory disease. on a normal day in the emergency room, they're seeing 100 kids. yesterday they saw 200. the reason for the increase, all the virus. >> we have had pretty severe patients. at least one, two, three, four severe patients with respiratory symptoms.