These viruses and bacteria enter the body with us when we breathe in the air or eat something. Once they are easily introduced into the body in this way, the viruses and bacteria begin to try to recruit new members.
However, when the body is healthy, it is strong enough to easily eliminate viruses and bacteria. However, when the body is weak, the resistance is also weak, and the body is unable to control them, resulting in a cold. A cold is when viruses and bacteria enter the body and begin to multiply.
By the way, viruses and bacteria can multiply in various places, such as the throat and bronchial tubes. Even though the body is defeated by these viruses and bacteria and becomes ill with a cold, the body is still fighting desperately against them at each location. Naturally, the body temperature rises and fever occurs. When viruses and other germs multiply in the throat, the throat nerves are stimulated and the throat suddenly starts to ache. The bronchial tubes try to expel the virus, and the person coughs. In other words, the fever, cough, and sore throat caused by a cold can be attributed to the body's battle against viruses and bacteria in each of these areas.

Influenza is a disease that occurs when a specially fertile pathogen called the influenza virus invades and multiplies.