Autism

Sarah Jane Smith's granddaughter Lily was autistic. This manifested in many ways, including her tendency to scream when prevented from doing something she loved, such as playing in the snow, or her habit of attacking other children or simply hitting them. (PROSE: Lily)

Ryan Scott, who was also autistic, would sing to isolate himself from the world. (PROSE: Into the Silence)

Olivia Jennings thought that the Sixth Doctor might be autistic. (AUDIO: The Curious Incident of the Doctor in the Night-Time)

Behind the scenes
Bethany Black, who appeared in Sleep No More, confirmed that she was autistic. (REF: THWR 250, THWR 251)