About Influenza | Influenza (Flu)

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Higher Risk </a> <a data-post-id="133542" href="/flu/prevention/index.html" tabindex="0"> Prevention </a> <a data-post-id="133754" href="/flu/vaccines/index.html" tabindex="0"> Vaccine Basics </a> <a data-post-id="133752" href="/flu/testing/index.html" tabindex="0"> Diagnosis </a> <a data-post-id="133541" href="/flu/treatment/index.html" tabindex="0"> Flu Treatment </a> <a class="view-all" href="/flu/site.html"> View All </a> </div> </div> </header> <main class="container cdc-main" role="main" aria-label="Main Content Area"> <section class="cdc-page-title-bar cdc-page-offset noindex"> <time class="cdc-page-title-bar__item cdc-page-title-bar__item--date" datetime="2024-08-13 04:00:00"> August 13, 2024 </time> <div data-alt-langs class="cdc-page-title-bar__item cdc-page-title-bar__item--languages"> <a class="cdc-page-title-bar__item" href="https://espanol.cdc.gov/enes/flu/about/index.html">Español</a> </div> </section> <div class="cdc-page-title cdc-page-offset syndicate"> <h1>About Influenza</h1> </div> <div id="content" class="cdc-page-content cdc-page-offset syndicate container"> <div class="cdc-dfe-body "> <div class="cdc-dfe-body__top "> <div class="dfe-section dfe-section--page-summary summary-list" data-section="cdc_disease_basics_page_summary"> <div class="dfe-section__header"> <h2 class="editor-block--title" id="heading-60sxeawahr">Key points</h2> </div> <div class="dfe-section__content"> <ul class="nested-list"> <li class="level-1">Flu is a contagious respiratory illness caused by influenza viruses<ul> </ul> </li> <li class="level-1">Flu can cause mild to severe illness<ul> </ul> </li> <li class="level-1">Most experts believe that flu viruses spread mainly by tiny droplets made when people with flu cough, sneeze, or talk<ul> </ul> </li> <li class="level-1">The first and most important step in preventing flu is to get a flu vaccine each year.<ul> </ul> </li> </ul> <div class="dfe-section__nav hide" id="dfe-section__nav"> <div class="dfe-section__nav__header" id="dfe-section__nav__header"> </div> <div class="dfe-section__nav-links" id="dfe-section__nav-links"> </div> </div> </div> <div class="dfe-section__image" id="page-summary-image"> <figure class="dfe-image dfe-image__large"><div class="image-container calculated-ratio-16-9 aspect-ratio-16-9 stretch"><img src="/flu/media/images/2024/08/wild-mild-general.jpg" alt='A flu vaccine can take flu from wild to mild. A kayaker in rapids is above "wild" and a rubber ducky is under "mild."' title="Wild to Mild General Public"></div></figure> </div> </div> </div> <div class="cdc-dfe-body__center "> <div class="dfe-section" data-section="cdc_disease_basics_overview"> <a class="onThisPageAnchor" id="cdc_disease_basics_overview-understanding-influenza" name="Understanding%20Influenza" title="Understanding Influenza"></a><h2 class="editor-block--title" id="heading-9l-b4tigce">Understanding Influenza</h2> <p>Flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at times can lead to death. The best way to prevent flu is by getting a flu <a href="/flu/prevent/keyfacts.htm" class="ce-selection">vaccine</a> each year.</p> </div> <div class="dfe-section" data-section="cdc_disease_basics_symptoms"> <a class="onThisPageAnchor" id="cdc_disease_basics_symptoms-symptoms" name="Symptoms" title="Symptoms"></a><h2 class="editor-block--title" id="heading--brl7auipy">Symptoms</h2> <p>Flu can cause mild to severe illness, and at times can lead to death. Flu symptoms usually come on suddenly. People who have flu often feel some or all of these signs and symptoms:</p> <ul class="nested-list"> <li class="level-1">fever* or feeling feverish/chills<ul> </ul> </li> <li class="level-1">cough<ul> </ul> </li> <li class="level-1">sore throat<ul> </ul> </li> <li class="level-1">runny or stuffy nose<ul> </ul> </li> <li class="level-1">muscle or body aches<ul> </ul> </li> <li class="level-1">headaches<ul> </ul> </li> <li class="level-1">fatigue (tiredness)<ul> </ul> </li> <li class="level-1">some people may have vomiting and diarrhea, though this is more common in children than adults.<ul> </ul> </li> </ul> <p>*It's important to note that not everyone with flu will have a fever.</p> <p><strong>Not everyone with flu has symptoms</strong> </p> <p>Some people with influenza virus infections do not develop any symptoms at all. A <a href="https://academic.oup.com/jid/advance-article/doi/10.1093/infdis/jiae591/7909881?login=true" class="tp-link-policy" rel="noreferrer" data-domain-ext="com">household study<span class="visually-hidden sr-only" aria-label=" external icon" role="img"></span><span class="cdc-icon-external cdc-link-icon" aria-hidden="true" alt="external icon"></span></a> conducted during the 2017-2023 flu seasons found that 8 percent of people who tested positive for flu did not have symptoms.</p> <div class="dfe-block dfe-block--keep_reading"><b>Keep Reading:</b> <a href="https://www.cdc.gov/flu/symptoms/flu-vs-covid19.htm">Similarities and Differences between Flu and COVID-19</a></div> </div> <div class="dfe-section" data-section="cdc_disease_basics_incub"> <a class="onThisPageAnchor" id="cdc_disease_basics_incub-how-long-it-takes-for-signs-to-show" name="How%20long%20it%20takes%20for%20signs%20to%20show" title="How long it takes for signs to show"></a><h2 class="editor-block--title" id="heading-461e35uzrd">How long it takes for signs to show</h2> <h3 id="heading-iwlwty8ol0">Period of Contagiousness</h3> <p>You may be able to spread flu to someone else before you know you are sick, as well as when you are sick with symptoms.</p> <ul class="nested-list"> <li class="level-1">People with flu are most contagious during the first three days of their illness.<ul> </ul> </li> <li class="level-1">Some otherwise healthy adults may be able to infect others beginning one day <strong>before </strong>symptoms develop and up to five to seven days <strong>after </strong>becoming sick.<ul> </ul> </li> <li class="level-1">Some people, including young children and people with weakened immune systems, may be contagious for longer periods of time.<ul> </ul> </li> </ul> <h3 id="heading-wnwlcshrvy">Onset of Symptoms</h3> <p>The time from when a person is exposed and infected with influenza virus to when symptoms begin is about two days but can range from about one to four days.</p> </div> <div class="dfe-section" data-section="cdc_disease_basics_population"> <a class="onThisPageAnchor" id="cdc_disease_basics_population-people-at-risk" name="People%20at%20risk" title="People at risk"></a><h2 class="editor-block--title" id="heading-byaolq73be">People at risk</h2> <p>Anyone can get flu (including healthy people), and serious problems related to flu can happen at any age, but some people are at <a href="/flu/highrisk/index.htm" class="">higher risk</a> of developing serious flu-related complications if they get sick. This includes people 65 years and older, people of any age with certain chronic medical conditions (such as asthma, diabetes, or heart disease), <a href="/bmi/index.html" class="">people with a body mass index (BMI) of 40 kg/m<sup>2</sup> or higher</a>, those who are pregnant, and children younger than five years.</p> </div> <div class="dfe-section" data-section="cdc_disease_basics_causes_risk_spread"> <a class="onThisPageAnchor" id="cdc_disease_basics_causes_risk_spread-how-it-spreads" name="How%20it%20spreads" title="How it spreads"></a><h2 class="editor-block--title" id="heading-rlll8ukdxv">How it spreads</h2> <p>Most experts believe that flu viruses spread mainly by tiny droplets made when people with flu cough, sneeze, or talk. These droplets can land in the mouths or noses of people who are nearby. Less often, a person might get flu by touching a surface or object that has flu virus on it and then touching their own mouth, nose or possibly their eyes.</p> </div> <div class="dfe-section" data-section="cdc_disease_basics_prevention"> <a class="onThisPageAnchor" id="cdc_disease_basics_prevention-prevention" name="Prevention" title="Prevention"></a><h2 class="editor-block--title" id="heading-eafaanph8r">Prevention</h2> <p>The first and most important step in preventing flu is to get a flu vaccine each year. Flu vaccine has been shown to reduce flu-related illnesses and the risk of serious flu complications that can result in hospitalization or even death. CDC also recommends everyday preventive actions (like staying away from people who are sick <a href="https://www.cdc.gov/respiratory-viruses/prevention/physical-distancing.html" class="">(distancing)</a>, <a href="/respiratory-viruses/prevention/hygiene.html" class="">covering coughs and sneezes, frequent handwashing</a>, and <a href="/respiratory-viruses/prevention/air-quality.html" class="">taking steps for cleaner air</a>) to help slow the spread of germs that cause respiratory (nose, throat, and lungs) illnesses like flu. More information is available about <a href="/respiratory-viruses/prevention/index.html" class="">core and additional prevention strategies</a>.</p> </div> <div class="dfe-section" data-section="cdc_disease_basics_quick_facts_callout_callout"> <a class="onThisPageAnchor" id="cdc_disease_basics_quick_facts_callout_callout-quick-facts" name="Quick%20facts" title="Quick facts"></a><h2 class="editor-block--title" id="heading-xp1urquldv">Quick facts</h2> <h3 id="heading-6rocc8d2eq">U.S annual flu infection rates</h3> <p><a href="https://academic.oup.com/cid/article/66/10/1511/4682599" class="tp-link-policy" rel="noreferrer" data-domain-ext="com">A 2018 CDC study published in Clinical Infectious Diseases<span class="visually-hidden sr-only" aria-label=" external icon" role="img"></span><span class="cdc-icon-external cdc-link-icon" aria-hidden="true" alt="external icon"></span></a> looked at the percentage of the U.S. population who got sick with flu using two different methods and compared the findings. Both methods had similar findings, which suggested that on average, about 8 percent of the U.S. population gets sick from flu each season, with a range of between 3 percent and 11 percent, depending on the season.</p> <h3 id="heading-6c_wcfooan">Groups most likely to get sick from flu</h3> <p>The same <a href="https://academic.oup.com/cid/article/66/10/1511/4682599" class="tp-link-policy" rel="noreferrer" data-domain-ext="com">CID study<span class="visually-hidden sr-only" aria-label=" external icon" role="img"></span><span class="cdc-icon-external cdc-link-icon" aria-hidden="true" alt="external icon"></span></a> found that children are most likely to get sick from flu and that people 65 and older are least likely to get sick from flu. Median incidence values (or attack rate) by age group were 9.3% for children 0-17 years, 8.8% for adults 18-64 years, and 3.9% for adults 65 years and older. This means that children younger than 18 are more than twice as likely to develop a symptomatic influenza virus infection than adults 65 and older.</p> <h3 id="heading-an4avixpf1">Estimating seasonal flu incidence</h3> <p>Influenza virus infection is very common, and the number of people infected each season can only be estimated because not everyone will seek medical care or get tested for flu. Statistical estimations are based on <a href="/flu-burden/php/about/how-cdc-estimates.html" class="">CDC-measured flu hospitalization rates</a> that are adjusted to estimate the total number of influenza virus infections in the United States for a given flu season.</p> <p>The estimates for the number of influenza virus infections are then divided by the census population to estimate how common influenza virus infections are in the population (called seasonal incidence or attack rate).</p> <h3 id="heading-cdnvei8dai">Impact of flu season severity on incidence of flu</h3> <p>The proportion of people who get sick with flu varies. <a href="https://academic.oup.com/cid/article/66/10/1511/4682599" class="tp-link-policy" rel="noreferrer" data-domain-ext="com">A paper published in CID<span class="visually-hidden sr-only" aria-label=" external icon" role="img"></span><span class="cdc-icon-external cdc-link-icon" aria-hidden="true" alt="external icon"></span></a> found that between 3 percent and 11 percent of the U.S. population gets infected and develops flu symptoms each year. The 3 percent estimate is from the 2011-2012 season, which was an H1N1-predominant season classified as being of low severity. The estimated incidence of flu illness during two seasons was around 11 percent; 2012-2013 was an H3N2-predominant season classified as being of moderate severity, while 2014-2015 was an H3N2 predominant season classified as being of high severity.</p> <div class="dfe-block dfe-block--cdcmodule cdc_page_element dfe-block--width-standard"><div class="dfe-block__inner"><div class="cdc-textblock"><table class="table table-bordered main|gray-l4 table-striped main|gray-l4 nein-scroll"> <caption class="caption-top"><strong>Table 1. Estimates of the Incidence of Symptomatic Influenza by Season and Age-Group, United States, 2010–2022</strong></caption> <thead class="main|tertiary"> <tr> <th class="p-3" rowspan="2" scope="col"><span class="d-block text-center"><strong>Season</strong></span></th> <th class="p-3" rowspan="2" scope="col"><span class="d-block text-center"><strong>Predominant Virus(es)</strong></span></th> <th class="p-3" rowspan="2" scope="col"><span class="d-block text-center"><strong>Season Severity</strong></span></th> <th colspan="5" scope="colgroup"><span class="d-block text-center"><strong>Incidence, %, by Age Group</strong></span></th> </tr> <tr> <th class="p-3" scope="col"><span class="d-block text-center"><strong>0-4 yrs</strong></span></th> <th class="p-3" scope="col"><span class="d-block text-center"><strong>5-17 yrs</strong></span></th> <th class="p-3" scope="col"><span class="d-block text-center"><strong>18-49 yrs</strong></span></th> <th class="p-3" scope="col"><span class="d-block text-center"><strong>50-64 yrs</strong></span></th> <th class="p-3" scope="col"><span class="d-block text-center"><strong>≥65 yrs</strong></span></th> </tr> </thead> <tbody> <tr> <th scope="row"><span class="d-block text-center">2010-11</span></th> <td><span class="d-block text-center">A/H3N2, A/H1N1pdm09</span></td> <td class="p-1"><span class="d-block text-center">Moderate</span></td> <td><span class="d-block text-center">13.7</span></td> <td><span class="d-block text-center">8.42</span></td> <td><span class="d-block text-center">5.5</span></td> <td><span class="d-block text-center">8.2</span></td> <td><span class="d-block text-center">4.5</span></td> </tr> <tr> <th scope="row"><span class="d-block text-center">2011-12</span></th> <td><span class="d-block text-center">A/H3N2</span></td> <td><span class="d-block text-center">Low</span></td> <td><span class="d-block text-center">4.7</span></td> <td><span class="d-block text-center">3.7</span></td> <td><span class="d-block text-center">2.6</span></td> <td><span class="d-block text-center">3.2</span></td> <td><span class="d-block text-center">2.3</span></td> </tr> <tr> <th scope="row"><span class="d-block text-center">2012-13</span></th> <td><span class="d-block text-center">A/H3N2</span></td> <td><span class="d-block text-center">Moderate</span></td> <td><span class="d-block text-center">17.8</span></td> <td><span class="d-block text-center">12.5</span></td> <td><span class="d-block text-center">8.4</span></td> <td><span class="d-block text-center">12.8</span></td> <td><span class="d-block text-center">9.7</span></td> </tr> <tr> <th scope="row"><span class="d-block text-center">2013-14</span></th> <td><span class="d-block text-center">A/H1N1pdm09</span></td> <td><span class="d-block text-center">Moderate</span></td> <td><span class="d-block text-center">12.7</span></td> <td><span class="d-block text-center">7.4</span></td> <td><span class="d-block text-center">9.6</span></td> <td><span class="d-block text-center">13.7</span></td> <td><span class="d-block text-center">3.8</span></td> </tr> <tr> <th scope="row"><span class="d-block text-center">2014-15</span></th> <td><span class="d-block text-center">A/H3N2</span></td> <td><span class="d-block text-center">High</span></td> <td><span class="d-block text-center">16.1</span></td> <td><span class="d-block text-center">11.9</span></td> <td><span class="d-block text-center">6.3</span></td> <td><span class="d-block text-center">11.6</span></td> <td><span class="d-block text-center">10.1</span></td> </tr> <tr> <th scope="row"><span class="d-block text-center"><strong>2015-16</strong></span></th> <td><span class="d-block text-center">A/H1N1pdm09</span></td> <td><span class="d-block text-center">Moderate</span></td> <td><span class="d-block text-center">11.0</span></td> <td><span class="d-block text-center">7.7</span></td> <td><span class="d-block text-center">6.7</span></td> <td><span class="d-block text-center">10.5</span></td> <td><span class="d-block text-center">2.9</span></td> </tr> <tr> <th scope="row"><span class="d-block text-center">2016-17</span></th> <td><span class="d-block text-center">A/H3N2</span></td> <td><span class="d-block text-center">Moderate</span></td> <td><span class="d-block text-center">11.9</span></td> <td><span class="d-block text-center">12.0</span></td> <td><span class="d-block text-center">6.8</span></td> <td><span class="d-block text-center">11.8</span></td> <td><span class="d-block text-center">7.4</span></td> </tr> <tr> <th scope="row"><span class="d-block text-center"><strong>2017-18</strong></span></th> <td><span class="d-block text-center">A/H3N2</span></td> <td><span class="d-block text-center">High</span></td> <td><span class="d-block text-center">17.1</span></td> <td><span class="d-block text-center">13.3</span></td> <td><span class="d-block text-center">9.9</span></td> <td><span class="d-block text-center">18.4</span></td> <td><span class="d-block text-center">10.1</span></td> </tr> <tr> <th scope="row"><span class="d-block text-center"><strong>2018-19</strong></span></th> <td><span class="d-block text-center">A/H1N1pdm09, A/H3N2</span></td> <td><span class="d-block text-center">Moderate</span></td> <td><span class="d-block text-center">15.2</span></td> <td><span class="d-block text-center">12.4</span></td> <td><span class="d-block text-center">7.1</span></td> <td><span class="d-block text-center">11.4</span></td> <td><span class="d-block text-center">4.3</span></td> </tr> <tr> <th scope="row"><span class="d-block text-center"><strong>2019-20</strong></span></th> <td><span class="d-block text-center">A/H1N1pdm09, B</span></td> <td><span class="d-block text-center">Moderate/High</span></td> <td><span class="d-block text-center">19.8</span></td> <td><span class="d-block text-center">14.5</span></td> <td><span class="d-block text-center">9.6</span></td> <td><span class="d-block text-center">12.9</span></td> <td><span class="d-block text-center">3.5</span></td> </tr> <tr> <th scope="row"><strong>2020-21*</strong></th> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> <td></td> </tr> <tr> <th scope="row"><strong>2021-22</strong></th> <td><span class="d-block text-center">A/H3N2</span></td> <td><span class="d-block text-center">Low</span></td> <td><span class="d-block text-center">4.6</span></td> <td><span class="d-block text-center">5.1</span></td> <td><span class="d-block text-center">2.6</span></td> <td><span class="d-block text-center">2.3</span></td> <td><span class="d-block text-center">1.0</span></td> </tr> <tr> <th scope="row"><span class="d-block text-center">Median</span></th> <td></td> <td></td> <td><span class="d-block text-center">13.7</span></td> <td><span class="d-block text-center">11.9</span></td> <td><span class="d-block text-center">6.8</span></td> <td><span class="d-block text-center">11.6</span></td> <td><span class="d-block text-center">4.3</span></td> </tr> </tbody> </table> <p>* The burden estimate for the 2020-2021 season was not calculated due to the uncharacteristically low level of flu activity that season.</p> </div> </div></div><h3 id="heading-m3uuyo1zr2">Complications of Flu</h3> <p><a href="/flu/signs-symptoms/index.html" class="">Complications of flu</a> can include bacterial pneumonia, ear infections, sinus infections and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes.</p> </div> <div class="dfe-section" data-section="cdc_disease_basics_testing_screening"> <a class="onThisPageAnchor" id="cdc_disease_basics_testing_screening-testing-and-diagnosis" name="Testing%20and%20diagnosis" title="Testing and diagnosis"></a><h2 class="editor-block--title" id="heading-gtzxahuqus">Testing and diagnosis</h2> <p>It is very difficult to distinguish flu from other viral or bacterial respiratory illnesses based on symptoms alone. There are tests available to diagnose flu.</p> <div class="dfe-block dfe-block--keep_reading"><b>Keep Reading:</b> <a href="/flu/testing/index.html">Diagnosis for Flu</a></div> </div> <div class="dfe-section" data-section="cdc_disease_basics_treatment"> <a class="onThisPageAnchor" id="cdc_disease_basics_treatment-treating-flu" name="Treating%20Flu" title="Treating Flu"></a><h2 class="editor-block--title" id="heading-qml9dz8-fp">Treating Flu</h2> <p>There are <a href="/flu/treatment/whatyoushould.htm" class="">flu antiviral drugs</a> that can be used to treat flu illness.</p> <div class="dfe-block dfe-block--keep_reading"><b>Keep Reading:</b> <a href="/flu/season/index.html">Information for the Flu Season</a></div> </div> <div class="dfe-section" data-section="cdc_disease_basics_res"> <a class="onThisPageAnchor" id="cdc_disease_basics_res-resources" name="Resources" title="Resources"></a><h2 class="editor-block--title" id="heading-kmddgrjnrq">Resources</h2> <p>Flu A viruses can be broken down into sub-types depending on the genes that make up the surface proteins. Over the course of a flu season, different types (A & B) and subtypes (only for flu A) of flu circulate and cause illness.</p> <ul class="nested-list"> <li class="level-1"><a href="/flu/about/viruses-types.html" class="ce-selection">Types of Flu Viruses</a> Flu A and B viruses are responsible for seasonal flu epidemics more commonly known as the flu season.<ul> </ul> </li> <li class="level-1"><a href="/flu/php/viruses/change.html" class="">How Flu Viruses Can Change</a> Flu viruses can change in two different ways—antigenic drift and antigenic shift.<ul> </ul> </li> <li class="level-1"><a href="/flu-in-animals/about/index.html" class="">Transmission of Flu Viruses from Animals to People</a> Flu A viruses also are found in many different animals, including ducks, chickens, pigs, horses, whales, and seals.<ul> <li class="level-2"><a href="/bird-flu/index.html" class="">Avian (Bird) Flu</a>: Information on flu viruses in birds<ul> </ul> </li> <li class="level-2"><a href="/flu-in-animals/about/canine-flu.html" class="">Canine (Dog) Flu</a>: Information on flu viruses in dogs<ul> </ul> </li> <li class="level-2"><a href="https://www.cdc.gov/flu/swineflu/index.htm" class="">Swine (Pig)/Variant Flu</a>: Information on flu viruses in pigs and human infections with flu viruses from pigs<ul> </ul> </li> </ul> </li> <li class="level-1"><a href="https://www.cdc.gov/flu/pandemic-resources/index.htm" class="">Pandemic Flu</a> A flu pandemic is a global outbreak of a new flu A virus. Learn more, including about past flu pandemics.<ul> </ul> </li> <li class="level-1"><a href="/flu-resources/php/resources/" class="">Images of Flu Viruses</a> Graphics of generic flu viruses<ul> </ul> </li> <li class="level-1"><a href="/flu/php/viruses/human-serology.html" class="">Human Serology and Flu</a> CDC conducts human serology work to improve seasonal flu vaccines and prepare against future flu pandemics.<ul> </ul> </li> <li class="level-1"><a href="/flu/php/viruses/antigenic.html" class="">Antigenic Characterization of Flu Viruses</a> CDC antigenically characterizes circulating flu viruses each year to monitor for changes and to help inform flu vaccine composition recommendations.<ul> </ul> </li> <li class="level-1"><a href="/flu/php/viruses/genetic-characterization.html" class="">Flu Virus Genome Sequencing and Genetic Characterization</a> CDC studies genetic changes in circulating flu viruses to support public health objectives.<ul> </ul> </li> <li class="level-1"><a href="/fluview/overview/index.html" class="">Expanding Viral Genomic Sequencing Infrastructure in the U.S.</a> Through a cooperative agreement with the <a href="https://www.aphl.org/Pages/default.aspx" class="tp-link-policy" rel="noreferrer" data-domain-ext="org">Association for Public Health Laboratories<span class="visually-hidden sr-only" aria-label=" external icon" role="img"></span><span class="cdc-icon-external cdc-link-icon" aria-hidden="true" alt="external icon"></span></a>, CDC provides funding and training to a network of state labs for them to do influenza sequencing.<ul> </ul> </li> <li class="level-1"><a href="https://archive.cdc.gov/#/details?url=https://www.cdc.gov/flu/about/advanced-molecular-detection.htm" class="ce-selection">Advanced Molecular Detection (AMD) and Flu</a> CDC's Advanced Molecular Detection (AMD) initiative uses new technology to study flu viruses faster and in more detail than ever before.<ul> </ul> </li> </ul> </div> </div> <div class="cdc-dfe-body__right page-right-rail"> <aside class="page-right-rail__static noindex" role="complementary"> <div class="on-this-page " data-sid=""> <h2 class="on-this-page__heading"> On This Page </h2> <div class="card-body"> <ul> <li><a href="#cdc_disease_basics_overview-understanding-influenza">Understanding Influenza</a></li> <li><a href="#cdc_disease_basics_symptoms-symptoms">Symptoms</a></li> <li><a href="#cdc_disease_basics_incub-how-long-it-takes-for-signs-to-show">How long it takes for signs to show</a></li> <li><a href="#cdc_disease_basics_population-people-at-risk">People at risk</a></li> <li><a href="#cdc_disease_basics_causes_risk_spread-how-it-spreads">How it spreads</a></li> <li><a href="#cdc_disease_basics_prevention-prevention">Prevention</a></li> <li><a href="#cdc_disease_basics_quick_facts_callout_callout-quick-facts">Quick facts</a></li> <li><a href="#cdc_disease_basics_testing_screening-testing-and-diagnosis">Testing and diagnosis</a></li> <li><a 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