Agatha "undisclosed illness"

RogerAckroyd

"Christie wrote her first short story, The House of Beauty while recovering in bed from an undisclosed illness." wikipedia says.

What was the illness? And why still keep it shtum? Does any biography divulge this?

ChristieFanForLife

I think I remember reading somewhere in a biography or multiple ones of Agatha Christie that the illness was influenza.