|New York County, New York, Certificate of Death no. 1585 (11 January 1919), Nina Herman, New York City Municipal Archives, New York.|
Bronchial pneumonia is often triggered in those whose immune systems have been weakened by diseases such as influenza. Nina's case occurred during the 1918-1920 flu pandemic in New York. According to Wikipedia, this influenza strain was particularly deadly to young adults between 20 and 40 and death often came, in fast moving cases, from pneumonia.
The main portion New York City's epidemic ran from about 15 September 1918 to 16 November. It continued, to a lesser extent, for several months. During that time, 147,000 cases of influenza and pneumonia were reported and 20,608 adults died.