Symptoms of Influenza B: What You Need to Know

Influenza B is a common seasonal virus that affects millions of people worldwide each year. Unlike the common cold, Influenza B tends to come on suddenly and with more intensity. One of the hallmark signs is a high fever, typically rising above 38.5°C (101.3°F). In many cases, body temperature can spike to 39°C (102.2°F) or even reach 40°C (104°F), especially during the initial phase of infection.

Common Systemic Symptoms of Influenza B

Alongside fever, individuals often experience a range of systemic symptoms caused by the body's immune response. These include chills, persistent headaches, and widespread muscle and joint pain. Many patients report extreme fatigue and a noticeable drop in appetite, which can last for several days. These so-called "toxic" symptoms—indicative of the body fighting off a viral invasion—are usually more pronounced in Influenza B compared to milder respiratory illnesses.

Respiratory Symptoms: Less Common but Still Possible

Unlike Influenza A, Influenza B is less likely to present with classic upper respiratory "catarrhal" symptoms such as sneezing or a runny nose. However, a subset of patients may still develop throat discomfort, dry cough, or mild irritation in the airways. Some individuals produce small amounts of white phlegm, particularly as the illness progresses.

When Bacterial Infections Complicate the Illness

If a secondary bacterial infection occurs, the nature of the cough may change. Patients might begin producing thick, discolored, or pus-filled sputum—a sign of possible bronchitis or pneumonia. In more severe cases, especially among older adults or those with underlying health conditions, shortness of breath and difficulty breathing may develop, indicating lower respiratory tract involvement.

Gastrointestinal Symptoms in Influenza B

Interestingly, Influenza B is more frequently associated with gastrointestinal issues than other flu strains. A notable number of patients, especially children, experience nausea, vomiting, abdominal pain, or diarrhea. While these symptoms may resemble a stomach virus, their presence alongside a high fever and body aches strongly suggests influenza rather than gastroenteritis alone.

When to Seek Medical Attention

Early diagnosis and treatment are crucial. If you or a loved one exhibits high fever, severe body aches, respiratory changes, or digestive distress during flu season, it's important to consult a healthcare provider promptly. Antiviral medications like oseltamivir (Tamiflu) are most effective when started within the first 48 hours of symptom onset.

Additionally, staying hydrated, resting adequately, and monitoring for worsening symptoms—such as chest pain, confusion, or persistent high fever—can help prevent complications. Vaccination remains the best defense against Influenza B, reducing both the risk of infection and the severity of illness if contracted.