What is "the Flu"?

What is "the Flu"?
Am J Respir Crit Care Med. 2020 Sep 15;:
Authors: Sockrider M
PMID: 32931305 [PubMed - as supplied by publisher]

Source: American Journal of Respiratory and Critical Care Medicine - September 14, 2020 Category: Respiratory Medicine Authors: Sockrider M Tags: Am J Respir Crit Care Med Source Type: research