During the 1920s, chlorine gas (the same stuff used as a chemical weapon in World War I) briefly became popular as a cure for the common cold.
The Rushville Daily Republican - Mar 26, 1926
Jennifer Ackerman explains what was going on in her book Ah-Choo! The Uncommon Life of Your Common Cold
The use of chlorine gas to cure the common cold was suggested by observations that men who worked in chlorine plants to manufacture the noxious gas during the war were remarkably free of colds and flu. The same was true of soldiers on the front lines exposed to the pungent, biting fumes of chlorine, compared with those in the rear. A hundred years earlier, physicians had noted that people who worked and lived in the vicinity of bleaching establishments had fewer respiratory infections than others.
Chlorine was thought to act as a kind of thorn-in-the-flesh therapy. Vedder (dubbed "the chemical warrior" by Time magazine) proposed that "the irritant action of chlorine stimulates the flow of secretion and cleanses the mucous surfaces," resulting in "productive coughing and blowing of the nose." Through its oxidizing action, the gas was also thought to rid the body of toxins and fuel the activity of white blood cells useful in the attack against offending microbes.
Practically overnight, chlorine therapy became a popular treatment for victims of colds, bronchitis, and whooping cough... For the general public, there was Chlorine Respirine, 50 treatments for $0.50 in a handy collapsible tube, each dose purported to "knock a cold in three hours."
Controlled experiments eventually debunked the idea that chlorine gas had any curative value. Though I'm not sure how to explain why people exposed to chlorine didn't seem to catch colds as often. Perhaps it was just mistaken, anecdotal evidence. Or perhaps the chlorine gas was sterilizing surfaces, helping to prevent the transmission of germs.