Viruses and bacteria enter the body with air and food. In other words, they enter the body through the mouth and nose.
Viruses and bacteria enter the body through the mouth, attacking the throat, stomach, and intestines. On the other hand, viruses and bacteria entering through the nose first attack the nose, then the bronchi, and finally the lungs. The term "catching a cold" means that the virus or bacteria have entered the body and begun to multiply.
When these viruses and bacteria attack the nose and begin to multiply, they irritate the mucous membrane of the wall at the back of the nose, called the nasal mucosa, causing an increase in the amount of runny nose. No matter how many times you blow your nose, the runny nose will continue to run.
In other words, runny nose is caused by viruses and bacteria attacking the mucous membranes of the nose.