The flu is a contagious respiratory disease that affects millions of Americans each year, according to the Centers for Disease Control and Prevention (CDC). Some people-including the elderly, young children, pregnant women and people with certain kinds of medical problems are at high risk for complications of flu. In most healthy adults, the disease is characterized by the abrupt onset of symptoms similar to but more severe than the common cold.
Constitutional symptoms reflect the response of the immune system to the infection. Constitutional symptoms of flu, according to Dr. Raphael Dolin in "Harrison's Principles of Internal Medicine," include headache, feverishness, chills, muscle pain, malaise and extreme fatigue. In many cases, symptoms come on so abruptly that patients are able to recount the exact time that they became ill. Fever typically ranges from 100.4 to 105.8 degrees F. Temperature usually rises rapidly during the first 24 hours of illness, followed by a gradual return to normal over two to three days. However, there are exceptions, notes Dr. Dolin. In some cases, fever may last as long as a week. In other cases, there may be no fever at all. Headache may be generalized or only in the forehead (frontal) area. Muscle pain can occur anywhere but is most commonly felt in the lower back and legs. Some people also complain of joint pain that develops later.
Respiratory symptoms of flu often become more prominent as constitutional symptoms start to resolve. According to the CDC, respiratory symptoms of flu include cough, sore throat and runny or stuffy nose. The cough associated with flu is usually dry, although it may be accompanied by small amounts of clear or blood-tinged sputum. In many cases, advises Dr. Dolin, cough is accompanied by chest pain. Cough that produces copious amounts of sputum or green or yellow sputum suggests other medical problems or bacterial infections. Respiratory symptoms of flu may last for up to one week. Respiratory symptoms that fail to improve or get better only to become suddenly worse should be evaluated by a physician.
Some patients, according to "Harrison's Principles of Internal Medicine" report ocular symptoms of flu. Eye symptoms may include pain on motion of the eyes, sensitivity to light (photophobia) and burning sensation. Eye symptoms of flu resolve without treatment. Patients with eyes that are very red, produce discharge or get worse instead of better should seek physician evaluation.
In adults, notes MedlinePlus, the flu is rarely accompanied by gastrointestinal symptoms such as vomiting and diarrhea. These symptoms, according to the CDC, are more common in children. There is, however, one notable exception. In a 2009 study in the "New England Journal of Medicine" nearly 40 percent of people with H1N1 flu or "swine" flu reported vomiting (25 percent), diarrhea (25 percent) or both (13 percent). For H1N1 flu, gastrointestinal symptoms were typically mild and presented after constitutional and respiratory symptoms were established.