Signs and Symptoms of the Flu (Influenza)
The flu is an acute illness, meaning that symptoms usually come on suddenly. Infection usually lasts about one week and most patients recover without medical treatment within two weeks.
Flu viruses affect the respiratory tract and cause headache, runny or stuffy nose (rhinitis); dry, persistent cough; and sore throat. In some cases, cough associated with the flu can last longer than 2 weeks. Influenza also causes systemic symptoms, including the following:
- Chills
- Extreme tiredness (fatigue)
- Fever (usually high)
- Malaise (generally feeling unwell)
- Muscle aches (especially in the legs and back)
Nausea, vomiting, diarrhea, and ear infection (otitis media) also may occur with the flu. These symptoms are more common in children. Viruses that affect the gastrointestinal tract (e.g., stomach, intestines) often cause nausea, vomiting, and diarrhea. These illnesses may be referred to as "stomach flu," but they are not caused by influenza viruses.
The flu can cause complications, especially in patients who have other conditions (e.g., heart disease, asthma, diabetes, cancer), in children younger than 2 years of age, and in people over the age of 65. The most common complication is pneumonia, which can be viral, bacterial, or mixed. Primary influenza viral pneumonia is the most serious and difficult-to-treat pulmonary complication.
Other complications include dehydration (occurs when fluid output is higher than fluid intake), bronchitis, and sinus infections (sinusitis). Young children can experience febrile seizures related to infection with the flu.
Serious complications include the following:
- Encephalopathy (altered brain function or structure)
- Myocarditis (inflammation of the middle layer of the heart wall)
- Myositis (muscle inflammation)
- Pericarditis (inflammation of the membrane that surrounds the heart [pericardium])
- Reye's syndrome
- Transverse myelitis (inflammation of the spinal cord)
Reye's syndrome, which is more common in children under the age of 15, can develop about 6 days after infection with the flu or another virus. It affects brain (encephalopathy) and liver (hepatomegaly) function and structure, and causes nausea and vomiting and changes in mental state (e.g., disorientation, agitation, seizure).
Reye's syndrome, which may be associated with the use of aspirin, is a serious illness and may cause death in as many as 80% of cases. The incidence of Reye's syndrome has declined in the United States Since due to warnings about using aspirin in children who have viral illnesses (e.g., flu, chickenpox).
Common Cold vs. Flu
The common cold is a respiratory illness that also is caused by a virus. It can be difficult to tell the difference between a cold and the flu on the basis of symptoms. In most cases, symptoms of the flu (e.g., fever, muscle aches, tiredness, cough) are more severe. Colds usually do not cause complications such as pneumonia or bacterial infections.
