CNN  —  Carbon monoxide, often called the “silent killer,” has no taste, odor or color, yet prolonged inhalation of the invisible gas has been tied to the recent deaths of several American tourists traveling abroad. In March, 14-year-old Miller Gardner , son of former New York Yankees player Brett Gardner, died on a family vacation at a Costa Rica beach resort. A month prior, three American women were discovered dead in their Belize hotel room. In both cases, authorities have said carbon