You probably won't be surprised to learn that the word contact is a relative of contiguous, but would you believe that contagion and contingent are too? All of those words derive from the Latin contingere, meaning "to have contact with." The words contact and contiguous are fairly easy to connect with contingere, but what of the other two? In its early use, contingent was a synonym of "touching," and if you remember that touching something can pollute it (and that another meaning of contingere was "to pollute"), then contagion logically ties in, too.