The average person gets a cold 2-3 times per year. Colds are caused by viruses, such as Rhinovirus, but there are over a 100 types of viruses that can cause cold symptoms.
Once we get a virus, the first symptoms include a burning feeling in the nose or throat, and then sneezing, congestion, and fatigue.
The lining of our nose and throat typically produces over a liter of mucus a day. Normally, the fluid is swept to the back of the throat by tiny hairs called cilia. But when you get a cold, the mucus changes consistency and becomes thicker, and a different, greenish color. We used to think the green color meant an infection that needed antibiotics, but it is actually just a part of the cold process, caused by a type of white blood cells called neutrophils rushing to the lining of our nose to fight infection.