Influenza B virus is a less common but still important cause of influenza infection, that is seen throughout the world. The clinical presentation is similar to all other human influenza viruses - fever, chills, cough, rhinitis, pharyngitis, and other constitutional symptoms. Because various complications can arise (most notably secondary bacterial pneumonia), an early diagnosis is imperative. In addition to a complete laboratory workup, various microbiological studies can be employed to confirm an influenza B virus infection.
Even though the "flu" has largely been attributed to the influenza A virus in terms of causality, the influenza B virus is responsible for approximately 20% of all cases  . Similarly to influenza A, the influenza B virus prevails globally, but it primarily causes large epidemics and not pandemics, as is the case with influenza A . In the majority of the cases, an influenza B virus infection causes a mild acute respiratory infection, characterized by a sudden onset of fever, chills, myalgia, headaches, and cough  . Other notable complaints (particularly in the pediatric population) are pharyngitis, rhinitis, and asthenia, whereas gastrointestinal irritation, conjunctivitis, earache, and lymphadenopathy might also be encountered . These manifestations persist for about a week, although constitutional symptoms can last more . In rare cases, complicated infections may arise, especially in those who suffer from chronic cardiorespiratory diseases or diabetes mellitus . In infants and younger children, laryngotracheobronchitis can be potentially serious, whereas pneumonia (mainly secondary bacterial, but also primary viral) with hemoptysis, dyspnea, edema, and cyanosis is one of the most severe and possibly life-threatening complications . Other studies report urinary tract infections and kidney damage, atrial fibrillation and myocarditis, as well as central nervous system (meningitis, encephalitis, seizures) abnormalities   .
Because of the numerous complications that may arise, physicians must conduct a thorough physical examination and obtain a detailed patient history in the presence of an acute respiratory infection, especially in the pediatric population. Firstly, the abrupt onset of symptoms in the typical period of the year (during the winter) is highly suggestive of influenza, something which must be further corroborated by laboratory studies. A complete blood count (CBC) mainly shows leukopenia, whereas creatine kinase (CK) levels might also be elevated if myalgia is reported . As soon as sufficient evidence is raised to pursue a diagnosis of influenza B infection, several microbiological tests are available  :