Upper respiratory symptoms from flu and colds can be very uncomfortable. The most common cause of colds and flu is the rhinovirus, which lives in nasal passages. It enters through the nostrils and passes to the back of the nose where it invades cells lining the upper area of the throat. Runny nose and sinus congestion are responses to a rhinovirus invasion. Nasal drainage increases as your body tries to get rid of the virus. Inflammation and increased blood flow to sinonasal tissues cause sinus congestion.