【The Cause of the Common Cold Is a Virus, Not Bacteria】
When a patient reports early symptoms of a cold and says, “I caught a cold today,” the cause is viral infection—not bacterial. Viruses can easily invade living cells and establish infection. In contrast, it is extremely difficult for bacteria to penetrate living cells.
Once a virus infects a cell, the immune system’s killer T cells destroy the infected cells.
Around four days after the onset of a cold, the epithelial cells lining the respiratory tract begin to be attacked by the immune system. As these cells and mucous membranes become damaged, bacteria can more easily adhere to and proliferate on them. By around the fifth day, bacteria may be detected in sputum cultures. This is a secondary infection that occurs following the viral cold.
In cases of pneumonia, the bacteria detected in sputum are not the initial cause. Rather, they are opportunistic pathogens that invade through the gaps in the body’s defenses created by the viral infection. It is the virus that first enters the body’s cells.
On the first day of a viral cold, antibiotics are ineffective. Viruses cannot be cultured from sputum. Even if pneumonia appears on a chest X-ray or CT scan, the shadows seen are not those of the virus itself.