Doctors first identified cases of unexplained muscle weakness and limb paralysis in children, which they called acute flaccid myelitis, or AFM, in 2014 — though in retrospect, sporadic cases showed up as early as 2009, said Kevin Messacar, an infectious disease specialist at Children’s Hospital Colorado [and associate professor of pediatrics at CU School of Medicine]. The hospital was one of the first to raise alarms that something unusual was going on.