I dunno, I rather feel that the original question of whether or not Hobbits get lung cancer is about the same as the question of whether or not the inhabitants of Middle-earth of all species ever feel the need to go to the bathroom (so to speak). Of course they do, but we never see it, because it's not really relevant to the story. One could take up the position of a friend's old Sunday school teacher, though: when she was a little kid, my friend drew a picture in Sunday school of Mary changing Baby Jesus' diapers (no anatomical correctness to quibble about, she was just a kid and had the drawing skills thereof
) and was taken to task by the teacher, who found it not innocent but offensive. Apparently it was the teacher's belief that Jesus didn't do such base human things as piddle in his nappies, a trait which she also felt carried on into adulthood. I might have thought my friend was kidding if I hadn't heard this same line of thought from other people in my own church when I was a kid.
Some Hobbits probably did get lung cancer; it's the world marred by Melkor, after all, and disease is not unknown among the mortals. Hobbit toughness may be greater than our own, and they may have been less prone to cancers of all kinds, but in the end, the evil Melkor wrought in the world by infusing his power into it would affect them. Like humans of our own past centuries, the Hobbits may have thought it was "consumption" (which was really tuberculosis, if I recall correctly, but acted as a catch-all phrase for diseases of the lungs that slowly killed the victim). I shall have to take a look in the letters to see if Tolkien had any opinion about this. He might've.