According to “Harrison’s Principles of Internal Medicine,” acute viral respiratory illness such as the common cold, flu and pneumonia are among the most common human diseases, accounting for half or more of all acute illnesses. The common cold, flu and pneumonia share many of the same symptoms, which can make it difficult to tell the difference. In addition, the diseases are not mutually exclusive; pneumonia can be a complication of both colds and the flu. However, there are differences between the diseases that can help you tell whether you just have a cold or a more serious illness.
According to the American Academy of Family Physicians (AAFP), the symptoms of a cold develop slowly while the symptoms of flu usually start suddenly. In fact, notes Dr. Raphael Dolin in “Harrison’s Principles of Internal Medicine,” patients with flu are often able to recount the exact time when they became sick. Pneumonia usually falls somewhere in between. Most patients with viral pneumonia notice a deterioration over the course of 12 to 36 hours, advises the American Lung Association.
According to “Harrison’s Principles of Internal Medicine,” in most cases, cold symptoms subside within four to nine days. The flu usually resolves somewhat faster--two to five days--although respiratory symptoms such as dry cough and sore throat can persist for up to two weeks. Pneumonia presents somewhat differently. In the case of viral pneumonia, patients usually report the onset of breathing problems coincident with the worsening of previous cold and flu symptoms. In the case of secondary bacterial pneumonia, when bacteria invade lungs weakened by colds or flu, a patient usually starts to feel better only to become acutely worse.
Colds, flu and pneumonia follow a spectrum of severity. According to the AAFP, although colds and flu share many of the same symptoms, colds are generally less severe. Also, while both colds and flu can make breathing uncomfortable or require more effort than normal, they rarely result in impaired oxygenation, while shortness of breath is one of the most common symptoms of pneumonia. Other symptoms of pneumonia, according to the American Lung Association, include sharp or stabbing chest pain with inhalation, excessive sweating, increased breathing and heart rate and blueness around the mouth and lips. Although colds and flu usually resolve without treatment, patients with pneumonia always need to see a doctor.