Flu in Adults

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Medical Author:

Steven Fine, MD, PhD

Medical Editor:

Mary Nettleman, MD, MS, MACP

Mary Nettleman, MD, MS, MACP

Mary D. Nettleman, MD, MS, MACP is the Chair of the Department of Medicine at Michigan State University. She is a graduate of Vanderbilt Medical School, and completed her residency in Internal Medicine and a fellowship in Infectious Diseases at Indiana University.

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Medical Editor:

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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• Flu in Adults Overview
• Flu in Adults Causes
• Flu in Adults Symptoms and Signs
• Flu vs. Cold
• When to Seek Medical Care
• Diagnosis of Flu in Adults
• Flu in Adults Treatment
• Flu in Adults Home Remedies
• Medications for Flu in Adults
• Flu in Adults Follow-up
• Prevention of Flu in Adults
• More Flu Prevention Tips
• Prognosis of Flu in Adults
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• Synonyms and Keywords
• Author and Editors
• Read more on Flu in Adults from Healthwise
• Cold & Flu FAQs
• Viewer Comments: Flu in Adults - Symptoms

Influenza (flu) is an acute infection of the airway tract in the nose and throat that can sometimes spread down into the lungs. It is the most frequent cause of acute respiratory illness and can affect people of all ages. It occurs every year mainly in late fall and early winter and in a widespread fashion, affecting many people of different ages at the same time. The peak season for the flu in the northern hemisphere is from November through March.

• Some also confuse the flu illness with the term stomach flu. The latter is an illness associated with nausea, vomiting, and diarrhea and can be caused by bacteria and viruses other than the influenza viruses. It is a stomach illness, whereas the flu is an illness of the respiratory system.
• About 5%-20% of people in the United States get the flu every year, and more than 200,000 are admitted to a hospital for complications related to the flu. Each year, from 3,000 to as many as 49,000 Americans die from these complications. About 90% of these deaths occur in those older than 65 years of age.

Three types of influenza viruses exist. Types A and B cause epidemics of severe respiratory illnesses known as "the flu," and type C causes a mild illness not associated with epidemics. Type A is divided into different subtypes based on the chemical structure of the virus. The H1N1 swine flu virus is a type A influenza virus. Type B is not divided into subtypes. Both type A and type B are responsible for the seasonal outbreaks of flu.

• Outbreaks occur more frequently in the winter months. Many factors may play a role in this seasonal pattern:
• The virus survives for longer periods indoors in winter because the relative humidity of indoor air is very low in comparison to the outside air.
• The virus may stay suspended in the air for prolonged periods and thus infect others by being inhaled. The virus droplets can also infect by landing on sensitive body areas such as the eyes, nose, or mouth.
• In winter, humans tend to be indoors more and thus have closer contact with each other, which makes it easier for the virus to spread.
• Flu outbreaks are classified as epidemics (occurring in a set geographical area) or pandemics (a worldwide occurrence). A flu pandemic can occur when a new influenza A virus emerges against which there is very little immunity already in the human population. Because there is little immunity, the new virus can spread from person to person very easily and can sicken more people. In 2009, a pandemic influenza strain began circulating called "novel" H1N1 or swine flu.
• Influenza is a highly contagious disease. The virus is spread when you either inhale infected droplets in the air (spread when an infected person coughs or sneezes) or when you come in direct contact with an infected person's secretions (for example, by kissing, sharing of handkerchiefs and other items, and through use of objects such as spoons and forks). Touching smooth surfaces, such as doorknobs, handles, and telephones, are other ways to transfer the virus to your hands, which may then contact the nose or mouth where the virus gets absorbed.
• A person with flu is contagious for up to seven days after the onset of the illness, although the virus can be detected in the secretions up to 24 hours before the onset of symptoms. Thus, an individual can transmit the virus one day before symptoms begin.
• In young children, the virus can still be spread in the secretions into the second week of illness.

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