There is an article on the CNN site today reporting evidence that dogs have an ability to detect cancer by their sense of smell.
There is a link in the CNN article to the following blog post.
Several readers have posted comments saying that they can also smell cancer on others, even in public places. One comments that when she smells cancer on a stranger she does not say anything because it would be socially awkward to suggest that the stranger see a physician. What do physicians who are dermatologists do when they see a mole on person that they they know from professional experience should be removed and sent to a lab for testing? My guess is they say nothing because to do so would probably violate some professional norm or law regarding self referral. We need diplomatic ways to signal concern to one another (including people we don't know) without creating unnecessary alarm or offense. It seems to me that the price of silent diplomacy is high.