No two words in the English language are so similar, and yet so different.
Classic means something of the highest quality or class in a genre: a “classic” automobile like the Rolls Royce, a “classic” movie like “Gone With The Wind” or a “classic” book like “War and Peace”.
Classical can refer to a specific period in history, specifically the Classical Age of Greece and Rome; or also, when referring to music and literature. Classical refers to the most enduring forms of that genre: Homer’s “Iliad”, or Beethoven’s 5th Symphony.
Not all classical writing is “classic”, and not all classic literature is “classical”. Which is why we have “Classic Rock”, but we will never have “Classical Rock ‘n Roll”.
So when your doctor says you have a “classic” case of measles, he is really saying you have a typical case of measles. You won’t be mentioned in any medical textbooks as having anything extraordinary. It can sound confusing, but it really isn’t:
Use Classic for: an example: a case, novel, work, or automobile.
Use Classical for: music, ballet, architecture, scholar, or a specific historical period.