This is the 8136th most frequent German word.
influenza
Influenza: A viral infection commonly known as the flu, causing respiratory symptoms.
Here, "Influenza" is used as a noun referring to the disease, the flu, commonly occurring during the colder months.
Die Influenza verbreitet sich in der kühlen Jahreszeit schnell.
The influenza spreads quickly during the cold season.
In this case, "Influenza" is again used as a noun, indicating a specific case of the flu that was diagnosed.
Der Arzt diagnostizierte bei mir eine Influenza.
The doctor diagnosed me with an influenza.
Here, "Influenza" is used in a context indicating the general idea of the flu as a preventable disease through a campaign.