Influenza (Ital., 'influence'; called in French la grippe), one of the class of diseases to which the term zymotic has been applied, has been long recognised by medical writers. The popular application of the name to any severe cold in the head is not sanctioned by medical authority. Cullen called it catarrhus e contagio, but although, in most cases, it closely resembles ordinary catarrh, it presents certain points of difference from that disease. In addition to the ordinary symptoms of catarrh, there is a sudden, early, and very striking debility and depression of spirits. This early debility is one of the most marked and characteristic signs of influenza. The mucous membranes (especially those of the respiratory organs) are much affected. The tongue is white and creamy, the sense of taste is lost, there is no appetite, the pulse is soft and weak, the skin, although at first hot and dry, soon becomes moist, and the patient complains of pains and soreness in various parts of the body.
In simple, uncomplicated cases convalescence supervenes in the course of a week or sooner; but influenza is very frequently conjoined with bronchitis or pneumonia, in which case it is much more persistent and dangerous. There is, moreover, an extreme proneness to relapse on the slightest exposure, even after the patient feels perfectly recovered.
Influenza affords an excellent example of an epidemic disease, a whole community being often attacked in the course of a few hours. From this it may be inferred that the occurrence of this disease is connected with some particular condition of the atmosphere, but what that condition is is not known. Not unfrequently influenza follows close upon a sudden thaw; sometimes it is preceded by thick, ill-smelling fogs; but hot and cold, wet and dry weather have all been attended by severe outbreaks of the disease. Like cholera, influenza generally, but by no means constantly, follows a westerly direction, or one from the south-east towards the north-west, and its course seems to be altogether independent of the surface currents of air, as it often travels against the prevailing wind.
The epidemic which prevailed during the winter of 1889-90 in most parts of the civilised world, the first of importance in Britain for nearly forty years, presented some points of difference from most of the previously recorded outbreaks. In particular, there was in many places a much larger proportion of cases without any catarrhal symptoms whatever than appears to have been observed before. Such cases present a close resemblance to Dengue (q.v.), and many observers have come to the conclusion that there is a much closer relation between the two diseases than has been hitherto supposed; while some believe that the epidemic in question was itself dengue modified by climate, and not influenza at all.
The most important point in the treatment of influenza is not to bleed the patient, or in any way to depress his vital powers. He should be kept in bed; his bowels should be gently opened, his skin slightly acted upon, if dry; and, if the cough be troublesome, a mustard-poultice should be applied to the chest, and an expectorant mixture prescribed. Antipyrin and antifebrin were during the epidemics of 1889-95 found very valuable in combating the feverishness and pain of the early stage. In persons of weak or broken-down constitutions, ammonia, beef-tea, and wine and water must be given from the outset. The debility that often remains for a considerable period after the establishment of convalescence is best met by the preparations of iron, quinine, and strychnia.
Few diseases increase the death-rate to such an extent as influenza; more, however, in consequence of the great number of persons who are attacked in a severe epidemic than in consequence of its danger in individual cases. See EPIDEMIC, GERM.