Transmission of Influenza
Flu viruses are easily transmitted through water droplets in the air when an infected person coughs or sneezes. Influenza can also spread through hand-to-hand contact and other personal contact (e.g., through sharing a glass).
Cold, dry weather allows the virus to live longer outside the body and crowded conditions allow the virus to spread more easily. The influenza virus usually enters the body through the nose or throat and causes symptoms 1–4 days later.
A person who has contracted the flu can spread the virus the day before developing symptoms and as many as 5–7 days afterward. Patients who have a severely compromised immune system (e.g., patients who have HIV/AIDS) can spread the flu virus for weeks or months.