The Spanish nurse being treated for ebola says she touched her face with a tainted protective glove after helping treat a man dying from the virus.
Theresa Romero, 40, is in quarantine in a Madrid hospital but told doctors she believes she made the mistake after cleaning up after the man.
Heath officials said she had twice entered the room of Spanish missionary Manuel Garcia Viejo – once to change his incontinence pad and then to retrieve items after he had died.
Mrs Romero is the first person to contract the virus outside Africa.
She earlier told El Mundo she had no idea how she was infected and that she had followed all precautions.