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From the convention of using "X" as a variable to stand for something unknown.

Proper noun[edit]


  1. Any of various diseases of plants or animals whose origin was at first a mystery, especially:
    1. Hyperkeratosis in cattle.
      • 1949, United States. Animal Industry Bureau, Report of the Preliminary Survey on "X" Disease, page 10:
        In making a diagnosis of X-disease, one must consider the insidious onset, slow course, and number of animals affected.
      • 1954, United States. Agricultural Research Service, Hyperkeratosis (X-disease) of Cattle, page 5:
        At the time of its first occurrence, nothing was known about the cause of X-disease or whether it could be treated and cured.
      • 1983, Comparison of Wood Preservatives in Stake Tests, page 128:
        Crankcase oils may contain chlorinated naphthalenes, which have been reported to contribute to "X-disease" (hyperkeratosis) in cattle.
      • 2016, Donald Roberts, Richard Tren, Roger Bate, The Excellent Powder: DDT's Political and Scientific History, page 187:
        For example, a 1940s claim attributed to DDT, without a shred of evidence, a mysterious cattle disease known as X-disease (discussed further in appendix 4).
    2. A disease of stone fruits caused by a form of mycoplasma.
      • 1977, The Plant Disease Reporter, page 1024:
        Transmission of X-disease was more erratic with peach than with chokecherry buds, possibly because only low titers of the X-disease mycoplasma-like organisms (MLO) are present in infected peach trees during most of the year.
      • 1998, Dissertation Abstracts International, page 4544:
        Studies of the phytoplasma associated with choke cherry X-disease in North Dakota.
      • 2012, Karl Maramorosch, Mycoplasma Diseases of Trees and Shrubs, page 35:
        A number of other yellows diseases of trees are of major importance throughout the world either because of local impact or widespread distribution. Among these are Prunus X-disease, pear decline, elm phloem necrosis, apple proliferation, the witches'—broom diseases of paulownia and jujube, and the greening and stubborn diseases of citrus.