Aragorn does not have time to identify and treat all the victims of the Black Shadow.
'There is not time, not time by half! I might treat all those suffering from the Black Shadow, but I fear I shall never find them before it is too late.
'Will you go before me, ${PLAYER}, and lay a marker by all those afflicted? These are the signs: a deep slumber and cold to the touch. Attend closely and you shall know them. The speed that would grant me may well be the difference between life and death.'
Find patients suffering from the Black Shadow in the High Hall, House of Remedies, or House of Rest, and mark them for Aragorn.
Aragorn stands in the High Hall of the Houses of Healing, on the Sixth Circle of Minas Tirith.
Return to Aragorn and let him know that you have marked out those suffering from the Black Shadow.