An example of increasing virulence from changing environmental conditions, is the experience in the United States Army camps in 1917 and 1918 with the streptococcus. This microorganism, which at first was but a secondary invader, particularly to measles, became so exalted in virulence that it soon became the cause of primary disease. This is likewise true of the various secondary invaders of the influenza epidemics. They become so highly virulent that they dominate the picture in the later stages. The organisms included in this group are particularly the streptococcus, the various pneumococci, and the meningococcus. Probably the tubercle bacillus should be added to this list. It requires a certain amount of time for such organisms to attain increased virulence. The earliest cases in any epidemic are comparatively very mild. Thus Major Billings, epidemiologist at Camp Custer, says that for the first five days of the autumn influenza epidemic in that camp the cases admitted to the hospital were very mild in character and were recorded as simple bronchitis and pharyngitis, of no great severity, the majority soon recovering. Five days after the first case was admitted, however, the entire symptom complex seemed to change, and the cases admitted to the hospital from then on were a very different and much more severe type. Major Billings, after going over the records, feels that both types of cases were the same disease, the second being a more severe form. Woolley reports essentially the same condition from Camp Devens. The same phenomena were found in 1889. During the 1889 epidemic Prudden examined by current bacteriologic methods seven cases of influenza and six cases of influenza-pneumonia. In them he found staphylococcus pyogenes aureus, streptococcus pyogenes, diplococcus pneumoniae, and in other cases he found a streptococcus. He concludes that the use of culture methods and media commonly employed has brought to light no living germ which there is reason to believe has anything to do with causing the disease. He emphasizes the probable importance of streptococcus pyogenes in particular in inducing the various complications.