late 14c., originally a medical term, from Medieval Latin ephemera (febris) "(fever) lasting a day," from fem. of ephemerus, from Greek ephemeros "daily, for the day," also "lasting or living only one day, short-lived," from epi "on" (see epi-) + hēmerai, dative of hēmera "day," from PIE *Hehmer "day." Sense extended 17c. to short-lived insects (Modern Latin ephemera musca) and flowers; general sense of "thing of transitory existence" is first attested 1751. Compare Greek ephemeroi "men," literally "creatures of a day."
"insect which lives for a very short time in its winged state," 1620s, from Greek (zōon) ephemeron, neuter of adjective ephemeros "living but a day" (see ephemera). Figurative use by 1771.
The use of the word by the Greek physicians apparently was from the notion of a fever rooted in the constitution of the body and symptomatic of one's physical condition, or else from its continuousness (compare ephemera). Hectic fevers are characterized by rapid pulse, flushed cheeks, hot skin, emaciation. In English applied particularly to the wasting fevers, rising and falling with the hours of the day, characteristic of tuberculosis.
Sense of "feverishly exciting, full of disorganized activity" is from 1904 and was a vogue word at first, according to Fowler, but hectic also was used in Middle English as a noun meaning "feverish desire, consuming passion" (early 15c.). Related: Hecticness.