Scully investigates a man who takes photographs of accident victims at the exact moment they die. Tithonus, in Greek mythology, was a mortal who fell in love with the goddess of dawn. The same Tithonus is also the titular character of a Tennyson poem . You might have noticed, however, that the prime suspect, Alfred Fellig, happens to be a photographer. Fellig tells Scully that he's cheated death ever since he miraculously survived a bout with yellow fever when he was young.
Yellow fever is a viral disease spread in humans by mosquito vectors.
Although it has currently resurged only in central Africa and South America, it did emerge in violent urban epidemics in America at several times during the 19th Century. Scientists have theories, but frankly they're not quite sure.