From the second edition (1989):
(ɪnfluːˈɛnzə) [a. It. influenza, lit. ‘influence’:—med.L. influentia: see influence.
It. influenza has the various senses of Eng. influence; but has, besides, developed (app. from the notion of ‘astral’ or ‘occult influence’) that of ‘visitation’ or ‘outbreak’ of any epidemic disease which assails many people at the same time and place (e.g. influenza di catarro, influenza di febbre scarlattina), a sense known as early as 1504; hence, absolutely, ‘an epidemic’; in 1743 applied specifically to ‘the epidemic’ (called also la grippe) which then raged in Italy, and spread over Europe generally, and for which the Italian word (anglicized in pronunciation) became the English specific name.]

A specific febrile zymotic disorder, highly contagious, and occurring for the most part in widespread epidemics. Its symptoms and sequelæ are extremely variable, but generally include rapid prostration and severe catarrh. The mortality is not high in proportion to the numbers attacked.
The term has been also applied loosely to any severe catarrh of the respiratory mucous membrane, esp. to a ‘cold in the head’ with running at the nose, sometimes called an influenza-cold. This use was frequent in the interval between the epidemic of 1847–8, and that which began in 1889, during which period no true influenza visited Great Britain.

1743 Lond. Mag. 145 News from Rome of a contagious Distemper raging there, call'd the Influenza. 1743 Mann Let. to Walpole 12 Feb. in Doran ‘Mann’ & Manners (1876) I. vi. 144 Everybody [in Rome] is ill of the Influenza, and many die. 1750 J. Huxham Fevers ii. (ed. 2) 20 The catarrhal Fever, which spread through all Europe under the Name of Influenza in the Spring 1743, frequently became pleuritic or peripneumonic. 1762 Mrs. Montagu in Doran Lady of last Cent. (1873) 133 Mr. Montagu‥had been much pulled down by the fashionable cold called l'influenza. 1770 Foote Lame Lover i. Wks. 1799 II. 62 Confin'd to bed two days with the new influenza. 1801 Nelson 5 June in Nicolas Disp. (1845) IV. 403 Sir Thomas Graves is still very ill‥In the St. George we have got the Influenza. 1803 Duncan Ann. Med. for 1802 II. ii. 480 The Influenza as it has appeared in Edinburgh in 1803‥has extended itself at different periods for near a thousand years past over almost the whole of Europe. 1831 P. Hawker Diary (1893) II. 29 Very unwell with the influenza that has, more or less, affected everyone this season. 1843 R. J. Graves Lect. Clin. Med. xxv. 543 In the portion of the nineteenth century already elapsed four influenzas have already occurred, viz., in 1803, 1831, 1834, and 1837. 1852 Theo. Thompson Ann. Influenza 2 In 1510, the first well described and widely prevalent epidemic of Influenza appeared. 1886 Fagge & Pye-Smith Princ. Med. (ed. 2) I. 1018 The practice, so common among the higher classes in this country, of designating as influenza any catarrhal attack that happens to be painful and distressing. 1892 F. A. Dixey Epidemic Influenza 1 During the first twenty-two weeks of 1890, 599 deaths were returned in London as primarily due to influenza‥[but] the tale of victims direct or indirect of this destructive malady cannot have fallen far short of 2800 for London alone.

b. A communicable disease of horses, characterized by shivering and fever, affection of the respiratory organs, and great weakness.

1872 Longfellow in Life (1891) III. 209 An influenza is raging among the horses.

c. fig. Applied to a mental or commercial epidemic; a prevalent craze; an attack of some general state of prostration.

1774 J. Bryant Mythol. I. 199 The learned Michaelis‥says, that it [the attempt to derive all words from Hebrew] is the reigning influenza, to which all are liable, who make the Hebrew their principal study. 1784 Gouv. Morris in Sparks Life & Writ. (1832) I. 268 The present influenza is the banko-mania. 1785 A. M. Bennett Juv. Indiscretions (1786) I. 153 Mr. Downes was certainly smitten with Lavinia Orthodox, but not with the matrimonial influenza. 1834 Southey Doctor xxiv. (1862) 56 Such preachers have never failed to appear during the prevalence of any religious influenza. 1891 Daily News 29 June 2/2 Some months ago the markets were said to be suffering from financial influenza.

d. attrib. and Comb., as influenza bacillus, etc.; influenza-cold, a severe cold with symptoms resembling those of influenza.

1891 C. Creighton Hist. Epidem. 570 A pure and unmistakable epidemic of influenza-cold. 1896 Daily News 15 July 5/3 More than one bacillus, closely allied to the influenza bacillus, but differing from it in some biological and microscopical features, has been found in seven out of eight cases of ‘influenza cold’. 1896 Allbutt's Syst. Med. I. 681 In some of these [patches of solid lung] the influenza bacillus has been found, thus shewing the disease in truth to be influenzal pneumonia. Ibid. 684 The chief characteristic of this influenza smell was its overpowering nastiness.

Hence inˈfluenzaed, -a'd, †influˈenzacized adjs., attacked by influenza; influˈenzaish a., having some of the qualities of influenza; influˈenzal, influˈenzic adjs., of or pertaining to influenza, characterized by influenza; influˈenzally adv., in an influenzal manner; influˈenzoid a., resembling or allied to influenza.

1803 Med. Jrnl. IX. 518 The influenzal epidemic of the present period, in no instance, loses either its catarrhal form or nature. 1825 Sporting Mag. XVI. 354 Dependent on an influenzal state of the atmosphere. 1836 J. Mitford in Lett. & Remin. (1891) 51, I was so influenza'd when your letter came, that I thought of nothing but warming pans. 1841 R. Oastler Fleet Papers I. No. 14. 105 The atmosphere is gloomy—and I am influenzaish. 1849 Lond. Jrnl. 9 June 212/2 The comfort and the consolation of the influenzacised florist. 1857 Dunglison Med. Lex. 497 Influenzoid‥Resembling influenza.—Dr. T. Thompson. 1887 Standard 17 June, The influenzic attack is disappearing. 1892 Nation (N.Y.) 14 Apr. 281/2 His Eminence Cardinal Sanfelice, is ‘influenzaed’, as is about every third person in Naples. 1897 Brit. Med. Jrnl. 20 Mar. 744/1 Ill influenzally. 1955 Sci. News Let. 19 Mar. 190/2 Meningitis may also be caused by a germ called Hemophilus influenzae. This form is called influenzal meningitis, but has nothing to do with influenza. 1969 Daily Tel. 18 Dec. 15/3 There had been a sharp increase this month in influenzal-type illness in South-East England. 1972 Ibid. 20 Jan. 7 A total of 180 people died during the week ending Jan. 7 from influenza and influenzal pneumonia.