Influenza is a contagious respiratory illness that affects millions of people worldwide every year, causing symptoms that range from mild discomfort to severe complications that can lead to hospitalization or even death.
Understanding Influenza
Influenza, commonly known as the flu, is a viral infection that attacks the nose, throat, and sometimes the lungs. It is caused by influenza viruses, which are different from the viruses that cause the common cold or stomach flu. While many people recover from the flu on their own within a week, this illness can become serious and even deadly for certain groups of people.[1]
The flu is not just a more severe cold. It typically comes on suddenly, and its symptoms can make you feel much worse than a cold would. When you catch the flu, you might feel so unwell that you cannot continue with your normal daily activities. The illness can force you to miss work or school and may require you to stay in bed for several days.[2]
There are four main types of influenza viruses: A, B, C, and D. Influenza A and B viruses are the ones that cause seasonal flu epidemics in humans, with most cases occurring during the winter months. Influenza C usually causes only mild illness and does not lead to epidemics. Influenza D primarily affects cattle and is not known to cause illness in people.[4]
How Common Is Influenza
Influenza is one of the most common infectious diseases affecting people around the world. Globally, there are approximately one billion cases of seasonal influenza every year, including between three and five million cases of severe illness. The disease causes an estimated 290,000 to 650,000 respiratory deaths annually.[4]
In the United States alone, about 20 to 40 million people catch the flu every flu season. The impact on children is particularly notable, with the disease affecting 20 to 30 percent of children compared to five to ten percent of adults each year.[1][3]
The burden of influenza is especially heavy in developing countries. Ninety-nine percent of deaths in children under five years of age with influenza-related lower respiratory tract infections occur in developing countries. Even in developed nations, people aged 65 years and older account for 50 to 70 percent of flu-related hospitalizations and 70 to 90 percent of flu-related deaths.[4]
Flu season in the Northern Hemisphere, which includes North America, typically runs from October through May, with the highest number of cases usually occurring between December and February. During the 2024-2025 flu season in the United States, preliminary estimates indicated at least 47 million flu-related illnesses, 21 million medical visits, 610,000 hospitalizations, and 27,000 deaths, including 281 pediatric deaths.[3][5]
What Causes Influenza
The flu is caused by infection with influenza viruses. These viruses are not the same as those that cause colds or stomach bugs. The influenza virus specifically targets your respiratory system, which includes your nose, throat, and lungs.[1]
Influenza A viruses are further divided into subtypes based on proteins found on their surface. Currently, the main subtypes circulating in humans are A(H1N1) and A(H3N2). The A(H1N1) subtype is also written as A(H1N1)pdm09 because it caused a pandemic in 2009 and replaced an earlier strain that had been circulating before that time. Aquatic birds are the primary natural source of influenza A viruses, though these viruses also spread widely among various mammals, including humans and pigs.[4]
Influenza B viruses are not classified into subtypes but belong to different lineages, specifically the B/Yamagata or B/Victoria lineage. Both influenza A and B viruses cause seasonal epidemics, though influenza A viruses are the only type known to have caused pandemics throughout history.[4]
The viruses spread primarily through tiny droplets that are made when people with flu cough, sneeze, or talk. These droplets can land in the mouths or noses of people who are nearby. Less commonly, 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 eyes.[1]
Who Is at Higher Risk
While anyone can get the flu, including healthy people, certain groups face a higher risk of developing serious flu-related complications if they become infected. Understanding who is most vulnerable helps these individuals take extra precautions and seek early treatment when needed.[1]
People aged 65 years and older are at significantly higher risk. As we age, our immune system naturally becomes less effective at fighting off infections, making older adults more susceptible to severe illness from the flu. Young children, especially those under five years old and particularly those under two years old, are also at elevated risk for complications.[3]
Pregnant women face increased risk of serious flu complications at any stage of pregnancy. The changes that occur in the immune system, heart, and lungs during pregnancy make expectant mothers more vulnerable to severe illness from influenza.[1]
People with certain chronic medical conditions are at higher risk regardless of their age. These conditions include asthma, chronic obstructive pulmonary disease (COPD), or other chronic lung diseases; heart disease or a history of stroke; diabetes, including gestational diabetes; kidney or liver disease; neurological conditions; blood disorders like sickle cell disease; and conditions that cause issues with muscle function or make it difficult to cough, swallow, or clear fluids from the airways.[3]
Individuals with weakened immune systems, whether from HIV/AIDS, cancer, or medications that suppress the immune system, are more likely to develop severe complications. People with a body mass index (BMI) of 40 or higher also face increased risk. Those who live in long-term care facilities or nursing homes are at higher risk because of their living environment and often because they have other health conditions.[1]
Young people under 19 years old who take aspirin regularly are at risk for a rare but serious complication called Reye’s syndrome if they get the flu. Additionally, research has shown that non-Hispanic Black people, non-Hispanic American Indians, Alaska Native people, and Hispanic or Latino people have higher rates of severe illness from the flu compared to non-Hispanic White people and non-Hispanic Asian people.[3]
Signs and Symptoms of Influenza
Flu symptoms usually come on suddenly, often within one to four days after you come in contact with the virus, with two days being the most typical timeframe. Unlike a cold, which tends to develop slowly, the flu can make you feel awful quite quickly.[1][4]
The most common symptoms of influenza include fever or feeling feverish with chills, though it is important to note that not everyone with flu will have a fever. A dry cough is very common, along with a sore throat and runny or stuffy nose. Many people experience muscle or body aches, which can be quite severe and affect the head, lower back, and legs in particular. Headaches are also frequent, as is overwhelming fatigue or tiredness that makes even simple activities feel exhausting.[1][2]
Some people, particularly children, may also experience vomiting and diarrhea, though these symptoms are less common in adults. The sudden onset is a key feature that helps distinguish flu from other respiratory illnesses. You might feel perfectly fine in the morning and then suddenly develop a high fever, chills, and severe body aches by the afternoon.[1]
Symptoms of the flu typically last about a week, though the most severe symptoms usually occur only for two to three days. However, fatigue and weakness can persist for several weeks after the other symptoms have resolved. A cough may also continue for one to two weeks after you start feeling better.[2][4]
Interestingly, not everyone infected with the flu virus develops symptoms. A household study conducted during the 2017-2023 flu seasons found that eight percent of people who tested positive for flu did not have any symptoms at all. However, even people without symptoms can still spread the virus to others.[1]
How to Prevent Influenza
The single most important step in preventing flu is getting vaccinated every year. The flu vaccine is updated annually to protect against the influenza viruses that research suggests will be most common during the upcoming season. Everyone six months of age and older should get a flu vaccine each year, preferably before flu season starts in early fall.[1][5]
The flu vaccine cannot give you the flu because it does not contain live virus capable of causing infection. Some people may experience mild discomfort or a slight fever after vaccination, but this is simply a sign that your body is responding to the vaccine and building protection. It is not the flu itself.[7]
For the 2025-2026 season in the United States, all flu vaccines are trivalent, meaning they protect against three strains of flu: two influenza A strains and one influenza B strain. The vaccine can vary in effectiveness from year to year, but even when it does not prevent infection completely, it can make the illness milder and help prevent serious complications that might require hospitalization.[5][7]
Beyond vaccination, there are several everyday preventive actions you can take. Washing your hands frequently with soap and water is one of the most effective ways to prevent the spread of flu. If soap and water are not available, use an alcohol-based hand sanitizer. Avoid touching your eyes, nose, and mouth, as germs spread easily this way.[18]
When you cough or sneeze, cover your mouth and nose with a tissue or use the upper part of your sleeve rather than your hands. This helps prevent the spread of droplets containing the virus. Avoid close contact with people who are sick, and if you become sick yourself, stay home to avoid spreading the virus to others.[18]
Clean and disinfect frequently touched surfaces at home, work, and school, especially when someone is sick. Maintaining healthy habits also supports your immune system: get plenty of sleep, stay physically active, manage stress, drink plenty of fluids, and eat nutritious food.[18]
How Influenza Affects the Body
When influenza viruses enter your body, they attach to cells in your respiratory tract and begin to replicate. The virus specifically targets cells lining your nose, throat, and airways. Once inside these cells, the virus takes over the cell’s machinery to make copies of itself, eventually destroying the infected cell as new virus particles are released to infect more cells.[6]
This process of infection and cell destruction triggers your body’s immune response. Your immune system recognizes the virus as a threat and launches a defense. Part of this defense includes inflammation in the infected areas, which contributes to many of the symptoms you experience, such as sore throat and congestion. The fever that often accompanies flu is actually your body’s attempt to create an environment less favorable for the virus to replicate.[6]
The muscle aches and fatigue that feel so debilitating during the flu are largely caused by chemicals called cytokines that your immune system releases to fight the infection. While these chemicals are necessary for fighting off the virus, they also cause inflammation throughout your body, leading to the characteristic body aches and extreme tiredness.[6]
In most healthy people, the immune system successfully clears the virus within about a week, and the damaged cells in the respiratory tract regenerate. However, the damage caused by the virus can sometimes allow bacteria to take hold, leading to secondary infections like pneumonia or sinus infections. This is particularly dangerous for people with weakened immune systems or chronic health conditions.[6]
In severe cases, influenza can spread deeper into the lungs, causing viral pneumonia. This occurs when the virus damages the delicate air sacs in the lungs, filling them with fluid and making it difficult to breathe. This type of pneumonia can lead to respiratory failure and is one of the most serious complications of influenza.[4]
Influenza can also worsen existing health conditions. For people with asthma, the flu can trigger severe asthma attacks. In people with heart disease, the stress of fighting the infection can lead to heart attacks or worsening heart failure. People with diabetes may find their blood sugar harder to control during a flu infection.[3]
