The Collaborative International Dictionary of English v.0.48:

malignant \ma*lig"nant\, a. [L. malignans, -antis, p. pr. of
   malignare, malignari, to do or make maliciously. See
   Malign, and cf. Benignant.]
   1. Disposed to do harm, inflict suffering, or cause distress;
      actuated by extreme malevolence or enmity; virulently
      inimical; bent on evil; malicious.
      [1913 Webster]

            A malignant and a turbaned Turk.      --Shak.
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   2. Characterized or caused by evil intentions; pernicious.
      "Malignant care." --Macaulay.
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            Some malignant power upon my life.    --Shak.
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            Something deleterious and malignant as his touch.
                                                  --Hawthorne.
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   3. (Med.) Tending to produce death; threatening a fatal
      issue; virulent; as, malignant diphtheria.
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   Malignant pustule (Med.), a very contagious disease
      produced by infection of subcutaneous tissues with the
      bacterium Bacillus anthracis. It is transmitted to man
      from animals and is characterized by the formation, at the
      point of reception of the infection, of a vesicle or
      pustule which first enlarges and then breaks down into an
      unhealthy ulcer. It is marked by profound exhaustion and
      often fatal. The disease in animals is called charbon;
      in man it is called cutaneous anthrax, and formerly was
      sometimes called simply anthrax.
      [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Pustule \Pus"tule\ (?; 135), n. [L. pustula, and pusula: cf. F.
   pustule.] (Med.)
   A vesicle or an elevation of the cuticle with an inflamed
   base, containing pus.
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   Malignant pustule. See under Malignant.
      [1913 Webster]

WordNet (r) 3.0 (2006):

malignant pustule
    n 1: a form of anthrax infection that begins as papule that
         becomes a vesicle and breaks with a discharge of toxins;
         symptoms of septicemia are severe with vomiting and high
         fever and profuse sweating; the infection is often fatal
         [syn: cutaneous anthrax, malignant pustule]