On May 15, a girl aged 6 years from Weld County awoke with symptoms of bilateral lower extremity weakness. She attended school as usual but needed assistance from a friend to walk outside for recess, where she fell down and was unable to get up. Her mother took her to an outpatient clinic, and a neurology appointment was arranged for the next day. She awoke the next day with a tingling sensation in her hands and feet, an inability to sit or stand on her own, and difficulty swallowing. She was taken to a local emergency department (ED) and transferred to a regional children's hospital. A physical examination revealed ophthalmoplegia (i.e., paralysis of muscles controlling eye movement), dysarthria (i.e., slurred or abnormal speech), and areflexia (i.e., absence of neurologic reflexes); nerve conduction studies indicated decreased velocities. The girl was admitted to the intensive-care unit on May 16 with a presumed diagnosis of Guillain-Barré syndrome and subsequently required intubation. On the evening of May 17, a nurse who was bathing the girl found a tick along her hairline. Investigators later learned that the tick had been visible on magnetic resonance imaging of the girl's head earlier that day. The tick was removed immediately, and the girl's symptoms improved; she was discharged home 1 week later. The tick was identified as a female Dermacentor andersoni. The girl often had visited her grandmother in the mountains in Larimer County and frequently hiked in the area. Seven days before symptom onset, the girl had visited her grandmother and played outside in the yard.