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""to make thin, to make less," 1520s, from Latin attenuatus, past participle of attenuare "to make thin, lessen, diminish," from assimilated form of ad "to" (see ad-) + tenuare "make thin," from tenuis "thin," from PIE root *ten- "to stretch." Related: Attenuated; attenuating. Earlier was Middle English attenuen "to make thin (in consistency)," early 15c." ])
(Etymology source: online etymology dictionary.)

The Merriam-Webster online dictionary defines "attenuate: thus:

"transitive verb

  1. "to lessen the amount, force, magnitude, or value of : weaken … shows great skill in the use of language to moderate or attenuate the impact of awkward facts. — Bernard Lewis
  2. "to reduce the severity, virulence, or vitality of an attenuated virus
  3. "to make thin or slender Glass can be attenuated into fibers.
  4. "to make thin in consistency : rarefy attenuate oil by heating it"

In vaccine development, viruses may be attenuated, often by "passage (a word derived from French - Frenchman Louis Pasteur was one of the pioneers of this process). Passaging, in this context, means growing the virus in a host (another animal) which to which it is not usually adapted; taking a sample of the virus in this host, and growing this sample on, again in a host (sometimes the same one) to which it is not usually adapted. Each time the virus is grown and collected from the new host is one pass in the "passage" process. After passaging the virus several times, a mutant version is developed which is not virulent in humans (or the animal in which the vaccine is to be used in veterinary practice), while retaining its ability to generate an effective immune response to "wild-type" viruses.

In recent times, other genetic technologies have been developed which allow organisms to be altered to create non-virulent strains.

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