What is Zeena's illness?

Zeena is described as asthmatic and sickly. Ethan questions how sick she could really be.... he believed a lot of her ailments were false.

Then the room grew perfectly black, and not a sound was audible but Zeena's asthmatic breathing.

Zeena had always been what Starkfield called “sickly,” and Frome had to admit that, if she were as ailing as she believed, she needed the help of a stronger arm than the one which lay so lightly in his during the night walks to the farm.