The Collaborative International Dictionary of English v.0.48:

Plague \Plague\, n. [L. plaga a blow, stroke, plague; akin to
   Gr. ?, fr. ? to strike; cf. L. plangere to strike, beat. Cf.
   Plaint.]
   1. That which smites, wounds, or troubles; a blow; a
      calamity; any afflictive evil or torment; a great trail or
      vexation. --Shak.
      [1913 Webster]

            And men blasphemed God for the plague of hail.
                                                  --Wyclif.
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            The different plague of each calamity. --Shak.
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   2. (Med.) An acute malignant contagious fever, that often
      prevails in Egypt, Syria, and Turkey, and has at times
      visited the large cities of Europe with frightful
      mortality; hence, any pestilence; as, the great London
      plague. "A plague upon the people fell." --Tennyson.
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   Cattle plague. See Rinderpest.

   Plague mark, Plague spot, a spot or mark of the plague;
      hence, a token of something incurable.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plague spot
    n 1: a spot on the skin characteristic of the plague