The phrase "flu-like symptoms" is a shorthand reference to a cluster of symptoms that typically occur with influenza, but present with another illness. Flu-like symptoms accompany a wide variety of infectious, autoimmune and other illnesses ranging from upper airway viruses to inhaled anthrax to cancer. The common thread that ties these conditions together and causes the flu-like symptoms is an activated immune response. Activated immune cells produce many chemicals, which induce the flu-like cluster of symptoms.
Fever
Fever occurs when the immune system or an infectious agent produce chemicals called pyrogens, which reset the body's temperature gauge producing fever. Your body has a temperature regulating system including a thermostat-like control center in the brain. Pyrogens turn up your body's thermostat and fever results.
Headache
Headache is a common complaint associated with many different types of illnesses. Flu-like headache is typically mild. It characteristically lacks what doctors call localizing signs and symptoms--meaning it is not accompanied by neurologic symptoms that point to a specific diseased area of the brain. Over-the-counter pain relievers such as acetaminophen, aspirin or ibuprofen often ease a flu-like headache.
Fatigue
Fatigue describes a condition of feeling chronically tired, worn out or lacking energy. The feeling may vary but persists despite rest and sleep. This symptom proves common and presents with many illnesses. Severe fatigue often interferes with the activities of daily life.
Malaise
You have almost certainly used the phrase, "I just don't feel well." Malaise is the word medical professionals use to describe this hard-to-define symptom. While it may prove difficult to put into words, malaise becomes easy to recognize when you experience it. This symptom often features prominently in flu-like illnesses.
Achiness
Achiness remains a common flu-like symptom. The feeling is characteristically body-wide as opposed to a specific muscle group or joint causing pain. Muscle achiness is typical, although the achiness may also include the joints. Over-the-counter pain relievers often ease generalized achiness.
Gastrointestinal Upset
Flu-like illnesses may include gastrointestinal symptoms such as loss of appetite, nausea, occasional vomiting or mild diarrhea. These symptoms often wax and wane and typically do not dominate the illness. This characteristic helps distinguish a flu-like illness from a gastrointestinal illness. Unintentional weight loss may accompany a protracted flu-like illness.
Runny or Stuffy Nose
Many non-influenza viruses cause a similar flu-like illness. As with true influenza, a runny or stuffy nose may feature prominently in these viral infections.
Dry Cough and Scratchy Throat
A non-influenza viral infection of the upper respiratory system commonly causes a dry cough and a scratchy or sore throat. The absence of phlegm (sputum) helps distinguish these flu-like illnesses from lung infections or other serious lung diseases.
References
- "Harrison's Principles of Internal Medicine, 16th Edition"; Dennis L. Kasper, M.D., et al, Editors; 2004
- "Principles and Practice of Infectious Diseases, Sixth Edition"; Gerald L. Mandell, M.D., et al, Editors; 2004
- Patient UK: Flu-like Illness
- American College of Physicians: Respiratory Tract Infections--How to Tell Cold or Flu Symptoms from Inhalational Anthrax
- Chemocare.com: Flu-like Syndrome
