Those who fell ill in medieval Europe were in trouble. Medieval physicians didn’t know about germs, infections, or, in many cases, the rudiments of human physiology. Medieval surgeons were barbers (who were busy men; in 1450, Parliament felt obliged to restrict English barbers to bloodletting, toothdrawing, cauterization, and "the tonsorial operations"). Doctors couldn’t prevent or cure many of the day-to-day maladies that could befall people in the best of times; when an epidemic hit, especially one as devastating and world-shaking as the fourteenth century black death that killed a third of Europe’s population, there was little left to do but pray. Several saints, most notably Saint Roch and Saint Sebastian, were thought to have particular power to intercede against plagues. When the going got really tough, they could turn to the Fourteen Holy Helpers, saints thought to be particularly effective at interceding on behalf of sufferers of different ills. The Holy Helper who dealt with plague was Saint Christopher. For hundreds of years, travellers carried Saint Christopher medallions. The legend of Christopher involved him being a sort of human ferryboat (he’s the patron saint of ferrymen, in addition to being the common replacement for Saint Botulf as patron saint of travellers). He carried a child across the river and was amazed by his weight, only to be informed that it was the Christ Child, who bore all the sins of the world upon His shoulders. Upon converting, his staff was turned into a tree; his emblems are therefore a staff, the Christ Child, and a tree. Not among his emblems is anything reflecting his giant stature; Christopher, the Golden Legend notes, was "of a right great stature, and had a terrible and fearful cheer and countenance. And he was twelve cubits of length…" (the language is Caxton’s). Twelve cubits is eighteen feet tall; it seems unlikely that the matter wouldn’t have come up. Nor, for that matter, does it seem likely that the Golden Legend would have neglected his dog’s head.
Saint Christopher was a cypher, a third century martyr upon whom an evolving myth could be overlayed. His true name is unknown; Christopher means "bearer of Christ", and his role as Christ’s literal bearer across the river is probably a back formation. In the Sixties, the Vatican took note of the bare modicum of facts known about his life and removed Christopher’s feast day from the universal calendar. Local parishes can celebrate Christopher, but it’s no longer required. The same thing happened to a number of the Fourteen Holy Helpers; Teresa Nielsen Hayden’s wonderful "Judging the Dubiousness of Saints" table docks points for having been on the list. Veneration of Saint Ursula was entirely suppressed on the grounds that there was no evidence she had ever existed.
It took a surprisingly long time for the process of sanctification to be formalized. The musician and visionary Hildegarde of Bingen (herself the author of two treatises on medicine) is often referred to as Saint Hildegarde and has a feast day, but has never been formally canonized, although she was investigated three times. Christopher’s holiness was proclaimed by members of the early church, and the tradition just grew from there. Why wouldn’t people imagine a saint who could protect them from plague? And Christopher was a martyr; people were willing to simply make up a Christian connection for the holy wells found throughout Europe (particularly associated with the Celtic saints). People will believe what fits their preconceived ideas, and they’ll believe it for a long time. Philomena was sanctified in the first half of the nineteenth century; her cult was in 1961 when Pope John XXIII noted that there was no evidence whatsoever that she had been martyred. Today, thousands flock to receive blessings from the unscrupulous (and doctrinally unsound) faith healer Benny Hinn. Some of them believe in his cures until the day they die. You could fill a museum with the various Perkins tractors, reflexophones, spectrochome therapy devices, and the dozens of other devices invented, all of which convey precisely the same meaning as the motto on the St. Christopher’s medallion: "PROTECT US".