The common cold is the most prevalent viral respiratory infection caused by different viruses; rhinoviruses (25%), coronaviruses (10%), parainfluenza viruses, respiratory syncytial virus (RSV), adenoviruses, and influenza viruses. It is the leading cause of patient visits to health care.
Rhinovirus, coronavirus, adenovirus, and many other viruses cause the common cold. A person can be infected by one virus serotype, recover, and have antibodies that protect from that serotype in the future; however, that person can be infected by another virus. Because of the involvement of multiple viruses and their serotypes, the common cold occurs numerous times.
You must be aware of how common it is. Everyone has some understanding or experience of the common cold. You might be suffering from a common cold right now or might have suffered earlier. This is the evidence to prove that the common cold is so common; the question arises, why is the common cold so common?
To answer this question, you need some background knowledge about host-pathogen interaction.
Each infectious disease results from a long battle between your immune system and the virulence factor (disease-causing capacity of) pathogens. When those pathogens overwhelm your immune system, you suffer from a disease, but if your immune system controls the pathogens, you become immune, or the resolution of the disease occurs.
To win this battle (host-pathogen interaction), your body uses various 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 them to hide from immune cells, or inactivate the effect of these cells.
As the common cold has multiple etiological agents, primary infection (an infection occurring for the first time) by each of these pathogens is possible. But antibodies and memory cells (formed after primary infection) are expected to protect us from these pathogens’ secondary infection. One of the immune system’s notable characteristics is that 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 but not others.
In the case of a common cold caused by rhinovirus, our body synthesizes antibodies and neutralizes or inactivates that virus whenever we get infected. Some antibodies are stored in our body to kill/neutralize if the same rhinovirus attacks our body again. But if the new rhinovirus (with a changed antigenic structure) attacks our body, already synthesized antibodies and memory cells may not recognize and remove/neutralize 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 continues.
Until now, virologists (scientists with profound knowledge about viruses) have found more than 100 types of such Rhinoviruses. This property of rhinovirus is called serotypes, and rhinovirus has more than 100 serotypes.
Another virus that also has multiple serotypes is the influenza virus. Now think, why can the influenza virus cause epidemics and pandemics?