Historical Review. 



Introduction. 



The most important advance in our ideas of the causation 

 of influenza was made in connection with the pandemic 1889 

 —92 when a number of different „miasmatic" theories had to 

 give way to the view that influenza was spread exclusively by 

 infection from man to man, and principally by direct trans- 

 mission from a patient to a healthy person. 



A singular and discouraging fate has however involved 

 our knowledge of the microbe of the disease. The influenza 

 pandemic referred to was the first which fell within the bac- 

 teriological era and it was a tempting task for medical bac- 

 teriology, still in its infancy, to discover the specific microbe 

 of the disease. R. Pfeiffer (1,2) also apparently succeeded 

 in completely raising the hitherto impenetrable veil which had 

 obscured the solution of the problem for numerous other ob- 

 servers, by immediately finding in the first cases of influenza 

 he investigated during a fresh outburst of the pandemic at 

 the end of November 1891, a small Gram-negative rod, con- 

 stantly present in large numbers, which on closer study proved 

 to have the previously unknown characteristic of only growing 

 on blood-containing or haemoglobin-containing media. The 

 proofs he advanced that this organism was really the virus 

 of influenza satisfied the requirements which were thought 

 necessary at that time, as many of the pitfalls of bacteriologi- 

 cal research which have later become evident were at that 

 date still unknown. 



Nevertheless the innumerable investigations on the bacterio- 

 logy of influenza which, especially during the last pandemic 



