4. Runny Nose

A runny nose results from an increase in the mucus secretion within the nasal cavity caused by immune responses to irritants and pathogens. This increase in mucus secretion serves to block harmful invaders such as viruses from entering the body, preventing them from causing disease and damage. Additionally, the increase in fluids serve to flush out particles that inadvertently find their way into the upper respiratory tract.

However, in the case of a flu infection, the continuous production of mucus eventually makes breathing difficult for a patient, causing them to cough or sneeze, transmitting the disease to other hosts.