Etymology
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disease (v.)

mid-14c., disesen, "to make uneasy, trouble; inflict pain," a sense now obsolete; late 14c. as "to have an illness or infection;" late 15c. in the transitive sense of "to infect with a disease, make ill;" from disease (n.). Tyndale (1526) has Thy doughter is deed, disease not the master where KJV has trouble not (Luke viii.49).

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infectious (adj.)
"catching, having the quality of spreading from person to person, communicable by infection," 1540s of diseases, 1610s of emotions, actions, etc.; see infection + -ous. Earlier in the same sense were infectuous (late 15c.), infective (late 14c.). Related: Infectiously; infectiousness.
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tongue (n.)

 Old English tunge "tongue, organ of speech; speech, a people's language," from Proto-Germanic *tungō (source also of Old Saxon and Old Norse tunga, Old Frisian tunge, Middle Dutch tonghe, Dutch tong, Old High German zunga, German Zunge, Gothic tuggo), from PIE root *dnghu- "tongue."

For substitution of -o- for -u-, see come. The spelling of the ending of the word apparently is a 14c. attempt to indicate proper pronunciation, but the result is "neither etymological nor phonetic, and is only in a very small degree historical" [OED]. In the "knowledge of a foreign language" sense in the Pentecostal miracle, from 1520s. Tongue-tied is first recorded 1520s. To hold (one's) tongue "refrain from speaking" was in Old English. Johnson has tonguepad "A great talker."

Bewar of tungis double and deceyuable,
Which with ther venym infect ech companye,
Ther poynaunt poisoun is so penetrable.
[John Lydgate, Fall of Princes (c. 1439)]
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vaccination (n.)

1800, used by British physician Edward Jenner (1749-1823) for the technique he publicized of preventing smallpox by injecting people with the similar but much milder cowpox virus (variolae vaccinae), from vaccine (adj.) "pertaining to cows, from cows" (1798), from Latin vaccinus "from cows," from vacca "cow," a word of uncertain origin. A mild case of cowpox rendered one immune thereafter to smallpox. "The use of the term for diseases other than smallpox is due to Pasteur" [OED].

The earlier 18c. method of smallpox protection in England was by a kind of inoculation called  variolation (from variola, the medical Latin word for "smallpox"). There are two forms of smallpox: a minor one that killed 2% or less of the people who got it, and a virulent form that had about a 30% mortality rate and typically left survivors with severe scarring and often blinded them. Those who got the minor form were noted to be immune thereafter to the worse. Doctors would deliberately infect healthy young patients with a local dose of the minor smallpox, usually resulting in a mild case of it at worst, to render them immune to the more deadly form. Jenner's method was safer, as it involved no smallpox exposure.

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bias (n.)

1520s, "oblique or diagonal line," from French biais "a slant, a slope, an oblique," also figuratively, "an expedient, means" (13c., originally in Old French a past-participle adjective, "sideways, askance, against the grain"), a word of unknown origin. Probably it came to French from Old Provençal biais, which has cognates in Old Catalan and Sardinian, and is possibly via Vulgar Latin *(e)bigassius from Greek epikarsios "athwart, crosswise, at an angle," from epi "upon" (see epi-) + karsios "oblique" (from PIE *krs-yo-, suffixed form of root *sker- (1) "to cut").

In the old game of bowls, it was a technical term used in reference to balls made with a greater weight on one side (1560s), causing them to curve toward one side; hence the figurative use "a one-sided tendency of the mind" (1570s), and, at first especially in law, "undue propensity or prejudice."

The bias of education, the bias of class-relationships, the bias of nationality, the political bias, the theological bias—these, added to the constitutional sympathies and antipathies, have much more influence in determining beliefs on social questions than has the small amount of evidence collected. [Herbert Spencer, "The Study of Sociology," 1873]
For what a man had rather were true he more readily believes. Therefore he rejects difficult things from impatience of research; sober things, because they narrow hope; the deeper things of nature, from superstition; the light of experience, from arrogance and pride, lest his mind should seem to be occupied with things mean and transitory; things not commonly believed, out of deference to the opinion of the vulgar. Numberless in short are the ways, and sometimes imperceptible, in which the affections colour and infect the understanding. [Francis Bacon, "Novum Organum," 1620]
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