From the Document: "Influenza ('flu') is a contagious disease that spreads around the United States every year, usually between October and May. Flu is caused by influenza viruses, and is spread mainly by coughing, sneezing, and close contact. Anyone can get flu. Flu strikes suddenly and can last several days. [...] Flu can also lead to pneumonia and blood infections, and cause diarrhea and seizures in children. If you have a medical condition, such as heart or lung disease, flu can make it worse."
Centers for Disease Control and Prevention: https://www.cdc.gov/