Common cold is caused by different viruses (not bacteria); rhinoviruses (25%), coronoaviruses (10%), parainfluenza viruses, respiratory syncytial virus, adenoviruses, and influenza viruses. Common cold is the most prevalent respiratory infection, leading cause of patient visits to health care, as well as school and work absenteeism.
You must be aware that how common it is. Everyone has some understanding or experience of the common cold. You might be suffering from common cold or might have suffered earlier. This is the evidence to prove that common cold is so common; so the question arises, So, why is common cold so common?
To answer this question you need some background information about host-pathogen interaction.
Every infectious disease from which we suffer is the result of a long battle between our immune system and the virulence factor (disease-causing capacity of) pathogens. When our immune system is defeated by those pathogens we suffer from a disease, but if our immune system gets a victory over pathogens, we become immune or the resolution of the disease occurs.
To win this battle (host-pathogen interaction) our body uses various arsenals (weapons) known as immune cells and their mediators such as T Cells, B Cells, antibodies, natural killer cells, interferons, cytokines, macrophages, etc. Similarly, pathogens also employ various virulence factors that either destroy these weapons (e.g., IgA protease), help to hide from immune cells or inactivate the effect of these cells.
As the common cold has multiple etiological agents, primary infection (an infection that is occurring the first time) by each of these pathogens is possible. But antibodies and memory cells (formed during/after primary infection) are expected to protect us from the secondary infection (second infection by the same pathogen) by these pathogens. One of the notable characteristics of the immune system is, the antibodies and memory cells are type-specific; i.e., the antibodies or memory cells formed against X pathogen, may protect us from that X pathogen only but not from others.
In the case of a common cold caused by rhinovirus, whenever we get infected, our body synthesizes antibodies and kills that virus (actually antibodies do not kill the virus, it recognizes the virus and binds with it, which facilitates it’s the removal/killing).
Some of these antibodies are stored in our body, to kill/neutralize it if the same rhinovirus attacks our body. But if the next rhinovirus with changed antigenic structure attacks our body, already synthesized antibodies and memory cells present in our body may not recognize and clear this virus. So we again suffer from a common cold.
Now our body synthesizes new antibodies for this new type of rhinovirus and the process goes on.
Till now scientists (virologists; a scientist with profound knowledge about viruses) been found more than 100 different types of such Rhinoviruses.
This property of rhinovirus is called serotypes and rhinovirus has more than 100 serotypes. A patient can be infected by one serotype of a virus, recover, and have antibodies that protect from infection by that serotype in the future; however, that person can be infected by another serotype of that virus. So because of the multiple serotype characteristics of rhinovirus, the infection caused by this virus is common.
Another virus that also has multiple serotypes is the influenza virus. Now think, why the influenza virus is able to cause epidemics and pandemics?