X-disease

In making a diagnosis of X-disease, one must consider the insidious onset, slow course, and number of animals affected.

Source: wiktionary

At the time of its first occurrence, nothing was known about the cause of X-disease or whether it could be treated and cured.

Source: wiktionary

Crankcase oils may contain chlorinated naphthalenes, which have been reported to contribute to "X-disease" (hyperkeratosis) in cattle.

Source: wiktionary

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).

Source: wiktionary