The common cold is a viral infection of the upper respiratory system. There are over 200 types of viruses that can cause a cold, and they are constantly evolving and changing. This explains why a vaccine or cure has not been developed, and also how someone can get another cold even though they have just recovered from one.  

It is possible, over time, to build immunity to some of the viruses that cause a cold, which is why adults tend to get fewer colds than children. In most cases, while the symptoms can make the person feel miserable, they are usually short-lived. However, for individuals with compromised immune systems – perhaps because of age (or lack of), having a long-term health condition such as asthma or diabetes, or being a smoker – the initial infection can lead to something more serious, such as acute bronchitis and pneumonia.