Influenza A is a serious respiratory illness that hospitalizes roughly 180,000 Americans and kills thousands in a typical season. For most healthy adults, it means a miserable week of fever, body aches, and exhaustion followed by a full recovery. But for certain groups, it can escalate into life-threatening complications, and even healthy people occasionally end up in the hospital.
What a Typical Case Looks Like
A standard bout of influenza A hits fast. Within one to two days of exposure, you develop a high fever, chills, muscle aches, a dry cough, sore throat, and deep fatigue. Unlike a cold, which builds gradually, the flu tends to flatten you all at once. Most healthy adults feel the worst during the first three to four days, start improving around day five, and return to normal within a week. Lingering fatigue and a dry cough can stick around for up to two weeks.
You’re contagious from about one day before symptoms appear until five to seven days after they start, which is why the flu spreads so efficiently through households, schools, and workplaces.
How Dangerous It Gets: Complications
The biggest fear with any flu case is pneumonia. Bacterial pneumonia develops when bacteria take hold in lungs already weakened by the virus. A large Japanese study of over 1.3 million influenza patients found that about 0.2% developed bacterial pneumonia within the first week, and another 0.09% developed it in the weeks after. Those percentages sound small, but applied to millions of flu cases per season, they translate to tens of thousands of pneumonia cases nationally.
The flu can also affect organs beyond the lungs. Heart inflammation (myocarditis) is a recognized complication, and the virus can trigger encephalopathy, a dangerous swelling of the brain. Among children who have died from influenza over the past 15 seasons, about 9% had encephalopathy or encephalitis listed as a factor. In some recent seasons, that figure reached 13 to 14%. These complications are rare in absolute terms, but they illustrate why influenza is not just a bad cold.
Who Faces the Greatest Risk
Certain people are far more likely to develop severe illness or die from influenza A. The highest-risk groups, as identified by the CDC, include:
- Very young children: Kids under 5 are at elevated risk, with the highest hospitalization and death rates among infants younger than 6 months.
- Adults 65 and older: Aging immune systems struggle to fight the virus, and existing health conditions compound the danger.
- Pregnant women: Elevated risk continues up to two weeks after delivery.
- People with chronic conditions: This includes asthma, COPD, diabetes, heart disease, kidney disease, liver disease, sickle cell disease, and neurological conditions.
- People with weakened immune systems: Whether from HIV, cancer, organ transplants, or immunosuppressive medications.
- People with a BMI of 40 or higher.
- Residents of nursing homes and long-term care facilities.
Racial disparities also play a role. Black, Hispanic, Latino, and American Indian or Alaska Native populations face higher hospitalization rates, driven by a combination of underlying health conditions and inequities in healthcare access.
Not All Influenza A Strains Are Equal
Influenza A comes in subtypes, and the two that circulate in humans are H1N1 and H3N2. They differ in severity in a way that surprises most people. H3N2 causes more total hospitalizations in a typical season, but H1N1 tends to make individual patients sicker. A CDC study found that compared to people hospitalized with H3N2, those hospitalized with H1N1 were 42% more likely to need intensive care, 79% more likely to need mechanical ventilation, and 25% more likely to die.
Which subtype dominates varies from year to year. In the most recent season with available data, H3N2 accounted for over 92% of subtyped influenza A viruses, with H1N1 making up less than 8%. A season dominated by H3N2 typically means more people get sick overall, while an H1N1-heavy season may produce fewer cases but more severe outcomes per case.
Influenza A vs. Influenza B
If you’re wondering whether influenza A is worse than influenza B, the answer is: not necessarily on a case-by-case basis. A CDC study comparing hospitalized adults found no significant difference in ICU admission rates, length of hospital stay, or death rates between the two types. Influenza B causes equally severe outcomes in individual patients. The reason influenza A gets more attention is that it causes more total infections and is responsible for pandemics, since it mutates more rapidly and can jump between animals and humans. Influenza B circulates only in people and changes more slowly.
How Much Vaccination Helps
The flu vaccine’s effectiveness shifts each year depending on how well it matches the circulating strains. For the 2024-2025 season, the vaccine reduced hospitalizations from H3N2 by roughly 51 to 55%, depending on age group. Against H1N1, it reduced hospitalizations in children by about 63%, with somewhat lower effectiveness in adults. These numbers mean the vaccine doesn’t eliminate your risk, but it substantially lowers your chances of ending up in the hospital. For people in high-risk groups, that reduction can be the difference between a rough week at home and a stay in the ICU.
Warning Signs That Need Immediate Attention
Most flu cases resolve on their own, but certain symptoms signal that the illness is turning dangerous. In adults, get emergency care for difficulty breathing, persistent chest or abdominal pain, confusion or inability to stay alert, seizures, not urinating, or severe weakness. A fever or cough that improves and then comes back worse is a classic sign of secondary bacterial pneumonia developing.
In children, watch for fast or labored breathing, bluish lips or face, ribs visibly pulling in with each breath, refusal to walk due to muscle pain, or signs of dehydration like no urination for eight hours or no tears when crying. A fever above 104°F that doesn’t respond to fever reducers warrants urgent care. For infants under 12 weeks, any fever during flu season is an emergency.
Putting the Numbers in Perspective
In a moderately severe flu season, the CDC estimates at least 15 million illnesses, 180,000 hospitalizations, and 7,400 deaths from influenza in the United States. Most of those deaths occur in adults over 65. Pediatric flu deaths, while far less common (17 were reported in the early weeks of the most recent season), receive outsized attention because they are largely preventable with vaccination and prompt treatment.
For a healthy adult, influenza A is almost always a self-limiting illness that resolves in a week. For a 70-year-old with diabetes, or a toddler, or someone on chemotherapy, it’s a genuine medical emergency. The seriousness of influenza A depends less on the virus itself and more on who it infects.