Wherever the specific bacilli are found, the diagnosis is certain. But even apart from this, the manifestations of influenza in the pronounced and typical varieties are so characteristic that the disease can generally be easily and certainly recognized. This is especially the case during the prevalence of large epidemics. Unquestionably during such times numerous diseases resembling influenza, which, if there were no epidemic, would certainly be given a different name, are called influenza, but the mistake is statistically insignificant in comparison with the enormous number of correct diagnoses, and is more than compensated by the fact that numerous complications of influenza, namely, pneumonia, pleurisy, etc., are not recognized as the results of influenza, but are registered as ordinary pneumonia, pleurisy, etc. A glance at the general mortality lists of penumonia, acute diseases of the respiratory organs, and tuberculosis in the pandemic periods will at once prove this point. (Compare p. 568.)
After the great epidemics, physicians, and more particularly the laity, are very prompt with their diagnosis of "influenza," so that the well known saying, "What one cannot bend," finds general application. Gradually the remembrance of the former great epidemics disappears, until finally the new generation of physicians has a practical interest only in the disease influenza nostras, or "la grippe," and for influenza vera merely a historic interest.
The differential diagnosis between a mild rudimentary influenza and intense coryza is possible only by means of a bacteriologic investigation. The same is true of the differential diagnosis between influenza nostras and influenza vera, especially in non epidemic periods. Clinically it is often as impossible to separate them as it is to distinguish cholera nostras from cholera Asiatica.
Yet there often are decisive clinical distinctions between influenza vera, on the one hand, and catarrhal fever, or "grip," on the other. A sudden onset with rigors and high initial fever, the severe prostration, the intensity of the nervous symptoms, the occasionally exclusive nervous character of the disease, without any respiratory symptoms, all point to influenza vera. If to these pneumonia is added in strong young individuals, the probability of the epidemic being influenza vera is much increased, because although endemic grip occasionally gives rise to a capillary bronchitis or catarrhal pneumonia, this occurs only in old, decrepit individuals and in small, weakly children. Finally, if the other severe complications are observed, especially in the respiratory system (abscess and gangrene of the lung or acute double pleurisy), in the cerebrospinal system (coma, meningitis, encephalitis, multiple neuritis, etc.), and in the digestive tract (hemorrhagic enteritis, etc.), every physician will recognize, even without a bacteriologic investigation, that he has to deal with an epidemic of influenza vera, even if the epidemic is local-that is to say, occurs during a non pandemic period. The bacteriologic demonstration of the specific influenza bacilli, provided that in the future, too, they remain the only proved cause of influenza vera, will add absolute certainty to the clinical diagnosis of such epidemics.