The secondary type of curve, that which is more frequently found in recurring influenza epidemics, characterized by a more gradual ascent, a still more gradual decline and a longer duration, is found frequently in the curves for other diseases; it is much less characteristic. We shall attempt by a comparison of epidemic influenza with these other diseases to explain the cause for this characteristic primary curve, so as to gain a further insight into the epidemic features of the disease. There are certain characteristics held by epidemic influenza in common with other diseases. There are certain resemblances between it and epidemic meningitis; in certain ways it resembles measles and there are some points of similarity to the pneumonic form of plague. The fact that it cannot be compared with one of these diseases to the exclusion of the others renders deductions more complicated. _Epizootics._—Soper has written at some length on a comparison of influenza in man with the so-called influenza among horses. The close resemblance in many features is striking. Epizootics of a disease apparently resembling influenza have been described among horses from before the Christian Era. A disease believed to have been influenza was recorded as having occurred B.C. among horses in Sicily. According to Parkes the epidemic which attacked the army of Charlemagne in 876 attacked at the same time dogs and birds. Finkler describes an epizootic among horses in 1404 A.D. There were other epizootics in 1301, 1711 and 1870 to 1873. In 1901 a severe outbreak occurred in America, and one has also been described by Mathers as occurring in Chicago in the winter of 1915–16. These epidemics of a disease clinically resembling influenza have frequently occurred among horses at the same time with true epidemics of influenza in man. Nevertheless there has been no clear cut evidence to prove that the disease is the same. Leichtenstern discusses the incidence of respiratory disease among animals, particularly household pets during epidemics of influenza. He comes to the conclusion that human influenza is a disease limited entirely to the human race and having no connection with animal disease. This is particularly true with regard to diseases reported among cats, dogs, canaries and other captive birds. He also believes that the epizootics among horses which have been reported from time to time as occurring with influenza epidemics have nothing to do with the disease in man. The symptoms are frequently very similar, but epizootics have frequently occurred at times when there was no epidemic of disease among humans. Abbott concluded that during the great horse epidemic of 1872 which bore a strong resemblance to influenza the disease was not unusually prevalent among men except in a few limited localities; while other infectious diseases, such as measles, small pox, scarlet fever and cholera infantum were unusually prevalent in that year.