Fever, I guess.

I did all I could." Duane saw now that the difference in her was a whiteness and tightness of skin, a hollowness of eye, a look of strain.
"Fever?
How long have we been here ?" he asked.
She took some pebbles from the crown of his sombrero and counted them.
"Nine.

Nine days," she answered.
"Nine days!" he exclaimed, incredulously.

But another look at her assured him that she meant what she said.

"I've been sick all the time?
You nursed me ?" "Yes." "Bland's men didn't come along here ?" "No." "Where are the horses ?" "I keep them grazing down in a gorge back of here.