With CF patients, they lack the water layer of the mucus in the lungs, making their mucus quite concentrated and sticky. This is because they lack the proteins to allow for chloride osmosis to the water layer, so overall there is less diffusion. As this makes the mucus more concentrated and thicker, it traps bacteria leading to more respiratory infections. Fick’s law does talk about short diffusion pathways across fluid membranes but it would be that as the mucus is so concentrated that it reduces diffusion rather than the distance travelled. It is easier for something to diffuse through liquid than a solid due to particle composition