Colds are caused by hundreds of different viruses. When one of these viruses gets into your body, your immune system attacks. Some of the effects of this immune response are the classic symptoms of a cold, such as congestion and coughing. The germs that cause colds are contagious. You can pick them up when an infected person sneezes, coughs, or shakes hands with you. After a couple of weeks, at the most, your immune system fights off the virus and you stop having symptoms. Allergies are caused by an overactive immune system. For some reason, your body mistakes harmless substances—such as mold or pollen—for germs and attacks them. Your body releases chemicals such as histamine, just as it does when fighting a cold. This can cause swelling in your nasal passages, a runny nose, coughing, and sneezing. Allergies are not contagious, although some people may inherit a tendency to develop them. The most telling distinction between Cold and Allergy are: presence of cough, sore throat and yellow mucus for cold. Allergy typically presents a runny nose with clear mucus.