Actually, I would use "probability" rather than "liability". Although you are using the word because the disease might be "liable" to go in an unexpected direction, "liability" usually means something disadvantageous (ie. his short stature was a liability when it came to playing basketball). To use "probability, the sentence would have to be reworded as:
"The probability of the disease taking an unexpected direction cannot be assessed"
You could also use the word "likelihood" ie:
"The likelihood of the disease taking an unexpected direction cannot be assessed"