Lion Heart by Paula Quinn

Tears are water for her roots and will make her stronger, not weaker.
Elias MacPherson, a warrior suffering the consequences of war falls in love with the compassionate heart and warm smiles of a lass who is a healing balm for his troubled soul. But she is a lass he can never have.
Lily Bennett is a forbidden woman who will not budge in her loyalty, though her heart is being swept away by the handsome Highlander who has come to save her world and make her laugh again. The Black Death, a force like nothing the earth has ever known, will bring them together and make them face their greatest fears.
Will they find themselves in the light that the other shines in the darkness? Or will an enemy, even more resilient than the plague, destroy them both?