You're sniffing, red nosed, and miserable. Is it something you picked up from your sneezing office mate? Or is it an allergy? Turns out it's pretty easy to tell the difference, once you know the signs. The common cold is caused by viral infections that target the upper respiratory system. These viruses are contagious and cause sneezing, coughing, a runny nose, itchy throat, and severe congestion. The mucus produced by a cold is usually thick, and green or yellow in color. Also, colds can bring on a fever and muscle achiness, which are uncharacteristic symptoms of allergies. And a cold usually lasts from seven to ten days, then it is gone. Allergies, on the other hand, are unique to each individual and are not contagious. Because the symptoms, which also include a runny nose, itchy eyes, and severe congestion, are caused by an allergen, there's no precise timetable for how long the symptoms will last and they can recur over and over. Also, allergies cause clear, watery mucus. One last difference - allergy sufferers tend to sneeze more frequently and in a choppier fashion, than people with colds, who sneeze heavily in order to purge the body of accumulating, thick mucus. To learn more about allergies, watch the other videos in this series.