These words have precise scientific meanings, but in everyday usage they are often confused. As for illness, a contagious disease is one communicated or transferred by contact; an infectious disease is indirectly communicated by such agencies as water and air. An infectious disease is not necessarily contagious. Contagious emphasizes the speed with which contagion (contact, communication, medium) spreads: "Contagious fear ran through the audience." Infectious suggests the powerful or irresistible quality of the source of contagion: "Mark Twain's infectious humor stimulated prolonged laughter and applause."
Dictionary of problem words and expressions. Harry Shaw. 1975.