Influenza B (flu B) causes the same core symptoms as other types of flu: sudden fever, chills, body aches, headache, sore throat, dry cough, runny nose, and deep fatigue. Symptoms come on abruptly, often within a matter of hours, and most people recover within one to two weeks. While flu B is sometimes described as “milder” than flu A, it can cause serious illness, particularly in children.
Common Flu B Symptoms
Flu B hits fast. Unlike a cold, which builds gradually over days, flu symptoms tend to arrive all at once. The hallmark signs include:
- Fever and chills, though not everyone develops a fever, especially older adults and people with weakened immune systems
- Body aches and muscle pain, often severe enough to make normal activity feel exhausting
- Headache
- Dry, nonproductive cough
- Sore throat
- Runny or stuffy nose
- Fatigue and malaise, which can linger well after other symptoms improve
Children with flu B are more likely than adults to experience gastrointestinal symptoms like nausea, vomiting, and diarrhea. Some children develop such severe muscle pain that they refuse to walk, which can be alarming but is a recognized flu symptom rather than a sign of a separate condition.
How Flu B Differs From Flu A
From a symptom standpoint, flu A and flu B are nearly identical. You can’t tell them apart based on how you feel. The real differences are epidemiological. Flu A circulates in both humans and animals, mutates more dramatically, and is responsible for flu pandemics. Flu B circulates only in humans, changes more slowly, and tends to hit hardest later in the flu season.
One notable clinical difference: a rare neurological complication called Reye syndrome is more closely associated with flu B than flu A. Reye syndrome has become very uncommon since the 1980s, when health authorities began discouraging aspirin use in children with viral illnesses. This is why children and teenagers with flu should never take aspirin or aspirin-containing products.
Timeline From Exposure to Recovery
Symptoms typically appear about two days after you’re exposed, though the incubation period can range from one to four days. Fever and body aches usually peak in the first two to three days of illness, then gradually improve. Cough and fatigue are the last symptoms to resolve and can stick around for two weeks or longer, even after you otherwise feel better.
You become contagious about one day before symptoms appear, which is part of why flu spreads so effectively. Most people remain contagious for five to seven days after getting sick, with the highest risk of spreading the virus during the first three days of illness. Young children and immunocompromised individuals may shed the virus for a longer window.
Complications to Watch For
Most healthy adults recover from flu B without complications. The greater concern is for young children, older adults, pregnant women, and people with chronic conditions like asthma or heart disease. The most common complications include pneumonia (a lung infection that can develop on top of the flu), dehydration, sinus infections, and ear infections. In children, flu can also trigger brain dysfunction such as encephalopathy or worsen preexisting conditions like asthma.
Certain warning signs indicate a need for immediate medical attention. In children, these include fast or labored breathing, bluish lips or face, ribs visibly pulling in with each breath, fever above 104°F that doesn’t respond to fever-reducing medication, dehydration (no urination for eight hours, dry mouth, no tears), seizures, or a fever or cough that improves and then suddenly worsens. In adults, the red flags are difficulty breathing, persistent chest or abdominal pain, confusion or dizziness, seizures, not urinating, and severe weakness. A fever or cough that gets better and then returns is a classic sign of a secondary infection like bacterial pneumonia developing on top of the original flu.
How Flu B Is Diagnosed
Rapid influenza tests, the kind you might get at an urgent care visit or doctor’s office, can identify whether you have flu A or flu B from a nasal swab. These tests return results in about 15 minutes, but they’re not perfect. For flu B specifically, rapid tests are required to detect at least 80% of true infections, meaning roughly one in five flu B cases can come back as a false negative. If the rapid test is negative but your symptoms strongly suggest flu, your provider may order a more sensitive molecular test (RT-PCR), which is significantly more accurate.
Testing matters most when the result would change your treatment plan, particularly if you’re in a high-risk group or it’s early enough in the illness for antiviral medication to help.
Treatment and Recovery
Antiviral medications work against flu B, but they’re most effective when started within the first 48 hours of symptoms. These medications can shorten the duration of illness by roughly one day and reduce the risk of complications. They’re most commonly recommended for people at higher risk of severe illness rather than every person who tests positive.
For most people, treatment is straightforward: rest, fluids, and over-the-counter medications to manage fever and pain. Acetaminophen or ibuprofen are appropriate fever reducers. Again, aspirin should never be given to children or teenagers with flu due to the risk of Reye syndrome. Staying hydrated is especially important for children, who are more vulnerable to dehydration from fever, vomiting, and reduced appetite.
Plan to stay home for at least 24 hours after your fever breaks without the help of fever-reducing medication. Given that you’re most contagious in the first three days, this helps protect the people around you while also giving your body the rest it needs to recover fully.