- Slideshow: Foods to Eat When You Have the Flu
- Slideshow: Natural Cold & Flu Remedies
- Slideshow: Finding Relief for Your Cough
- Patient Comments: Flu (Influenza) - Symptoms
- Patient Comments: Flu (Influenza) - Side Effects
- Flu (influenza, conventional, H1N1, H3N2, and bird flu [H5N1]) facts
- What is flu (influenza)?
- When does flu season begin and end?
- Flu vs. cold
- What are the causes of the flu (influenza)?
- What are flu (influenza) symptoms in adults and in children?
- What is the incubation period for the flu?
- How long is the flu contagious, and how long does the flu last?
- How is the flu (influenza) diagnosed?
- How does flu spread?
- What is the key to flu (influenza) prevention?
- Are there any flu shot or nasal spray vaccine side effects in adults or in children?
- How effective is the flu vaccine?
- Why should the flu (influenza) vaccine be taken every year?
- What are some flu treatments an individual can do at home (home remedies)?
- What types of doctors treat the flu?
- What can people eat when they have the flu?
- When should a person go to the emergency department for the flu?
- Who should receive the flu vaccine, and who has the highest risk factors? When should someone get the flu shot?
- What is the prognosis for patients who get the flu? What are possible complications of the flu?
- Can the flu be deadly?
- What is the bird (avian) flu?
- Do antiviral agents protect people from the flu?
- What medications treat the flu?
- Is it safe to get a flu shot that contains thimerosal?
- Where can people find additional information about the flu?
Quick GuideFlu Pictures Slideshow: 10 Foods to Eat When You Have the Flu
What is flu (influenza)?
Influenza, commonly called "the flu," is an illness caused by RNA viruses that infect the respiratory tract of many animals, birds, and humans. In most people, the infection results in the person getting a fever, cough, headache, and malaise (tired, no energy); some people also may develop a sore throat, nausea, vomiting, and diarrhea. The majority of individuals has symptoms for about one to two weeks and then recovers with no problems. However, compared with most other viral respiratory infections, such as the common cold, influenza (flu) infection can cause a more severe illness with a mortality rate (death rate) of about 0.1% of people who are infected with the virus.
The above is the usual situation for the yearly occurring "conventional" or "seasonal" flu strains. However, there are situations in which some flu outbreaks are severe. These severe outbreaks occur when a portion of the human population is exposed to a flu strain against which the population has little or no immunity because the virus has become altered in a significant way. These outbreaks are usually termed epidemics. Unusually severe worldwide outbreaks (pandemics) have occurred several times in the last hundred years since influenza virus was identified in 1933. By an examination of preserved tissue, the worst influenza pandemic (also termed the Spanish flu or Spanish influenza) occurred in 1918 when the virus caused between 40-100 million deaths worldwide, with a mortality rate estimated to range from 2%-20%.
In April 2009, a new influenza strain against which the world population has little or no immunity was isolated from humans in Mexico. It quickly spread throughout the world so fast that the WHO declared this new flu strain (first termed novel H1N1 influenza A swine flu, often later shortened to H1N1 or swine flu) as the cause of a pandemic on June 11, 2009. This was the first declared flu pandemic in 41 years. Fortunately, there was a worldwide response that included vaccine production, good hygiene practices (especially hand washing) were emphasized, and the virus (H1N1) caused far less morbidity and mortality than was expected and predicted. The WHO declared the pandemic's end on Aug. 10, 2010, because it no longer fit into the WHO's criteria for a pandemic.
A new influenza strain, H3N2, was identified in 2011, but this strain has caused only about 330 infections with one death in the U.S. Another strain, H5N1, a bird flu virus, has been identified since 2003 and has caused about 650 human infections; this virus has not been detected in the U.S. and currently is not known to be easily spread among people in contrast to other flu strains. Unfortunately, people infected with H5N1 have a high death rate (about 60% of infected people die); currently, H5N1 is not readily transferred from person to person like other flu viruses.
Haemophilus influenzae is a bacterium that was incorrectly considered to cause the flu until the virus was demonstrated to be the correct cause in 1933. This bacterium can cause lung infections in infants and children, and it occasionally causes ear, eye, sinus, joint, and a few other infections, but it does not cause the flu.
Another confusing term is stomach flu. This term refers to a gastrointestinal tract infection, not a respiratory infection like influenza (flu); stomach flu (gastroenteritis) is not caused by influenza viruses.
Although initially symptoms of influenza may mimic those of a cold, influenza is more debilitating with symptoms of fatigue, fever, and respiratory congestion. Colds can be caused by over 100 different virus types, but only influenza viruses (and subtypes) A, B, and C cause the flu. In addition, colds do not lead to life-threatening illnesses like pneumonia, but severe infections with influenza viruses can lead to pneumonia or even death.