Why is common cold so common?

Common cold is caused by different viruses (not bacteria); rhinoviruses (25%), coronoaviruses (10%), parainfluenza viruses, respiratory syncytial virus, adenoviruses and influenza viruses. Common colds are the most prevalent respiratory infections, leading cause of patient visits to health care, as well as school and work absenteeism.

You must be aware that how common it is. Every one have some understanding or experience about 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 diseases from which we suffer are 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 disease, but if our immune system get victory over pathogens, we become immune or the resolution of 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 (read above), primary infection (infection that is occurring 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 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 case of common cold caused by Rhinovirus, whenever we get infected, our body synthesize antibodies and kills that virus (Actually Antibodies does not kill the virus, it recognize the virus and binds with them, which facilitate the removal/killing of viruses).

Some of these antibodies are stored in our body so that whenever the next same rhino viruses attack our body, they can kill them. But if the next Rhinovirus with changed structure (antigenic variation) attacks our body, already synthesized antibodies and memory cells present in our body may not recognize and clear these virus. So we again suffer from common cold.

Now our body synthesize new antibodies for this new type of rhinovirus…. and the process goes on.
Till now scientists (virologists; 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. Patient can be infected by one serotype of 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 this multiple serotype characteristics of Rhinovirus, infection caused by this virus is common.

Another virus which also have multiple serotypes is Influenza virus. Now think, why influenza virus is able to cause epidemics and pandemics?