It is impossible to talk about colds without discussing viruses, since these ubiquitous organisms are the cause. Viruses are submicroscopic living particles, smaller than bacteria, that can infect any part of the body and cause symptoms. The lungs and respiratory tract are particularly vulnerable since viruses are easily passed from person to person in the air.
Viruses can cause virtually any type of respiratory infection.
The most benign is the common cold. In the United States viral infections of the respiratory tract are responsible for an estimated 250 million lost work days annually. One estimate puts the economic loss at 5 billion dollars each year, of which the common cold accounts for approximately 2 billion dollars. The common cold, bane of millions, is the most frequent viral infection of the respiratory tract. Not all symptoms are present every time.