Influenza pneumonia is a life-threatening complication of influenza virus infection, one of the most frequently encountered infections in clinical practice. After typical symptoms of upper respiratory tract infection and fever appear, severe respiratory decline with hypoxia and cyanosis can ensue without early recognition and appropriate therapy. Several diagnostic tests exist for prompt confirmation of influenza infection, but clinical suspicion is the key to diagnose the condition.
Influenza is a rather common viral upper respiratory tract infection among humans. It is caused by influenza A (and less commonly B) viruses, and the pathogenesis, as well as transmission, involves several animal hosts and human-to-human spread through air droplets via inhalation, coughing, or sneezing  . After an incubation period of a few days, constitutional symptoms of myalgia, fatigue, malaise, anorexia, fever, and a dry cough are main manifestations of influenza infection and usually last for 7-10 days   . It must be noted that the majority of infections are seen during the winter, and rarely in other parts of the year when pandemic or epidemic outbreaks occur   . In some patients, the infection is self-limiting, but numerous complications may arise, one of them being influenza pneumonia      . It is regarded as the most severe complication and appears more frequently in the presence of various risk factors . In children, concomitant cardiorespiratory illnesses (eg. asthma), diabetes mellitus, immunosuppression, long-term use of aspirin, and other diseases that increase the risk of hypertension are well-established risk factors, whereas pregnancy, chronic diseases, obesity, and residency in chronic care facilities are strongly associated with the development of influenza complications in adults and the elderly   . Influenza pneumonia represents the dissemination of the virus into the lower respiratory tract and the lung parenchyma, but secondary bacterial infections of the lungs might also be the culprit in influenza-infected patients, as the virus indirectly promotes bacterial adhesion to the respiratory epithelium     . For this reason, a clear distinction between primary influenza pneumonia and secondary bacterial pneumonia may be difficult to make. Nevertheless, approximately 2-5 days after the appearance of signs and symptoms typical for influenza, dyspnea, hypoxia, hemoptysis, cyanosis, and rapidly ensuing acute respiratory distress are hallmarks of influenza pneumonia    . In addition, cognitive impairment, gastrointestinal complaints (diarrhea, abdominal pain) and myalgia are reported, and are sometimes the only symptoms, particularly in the elderly population . In fact, pneumonia is one of the most important causes of death from this viral infection, and fatal outcomes might be seen within 24 hours    .
Mortality rates of influenza pneumonia are very high without early therapy, thus a detailed and comprehensive clinical, radiological, and microbiological assessment is vital in preventing the onset of respiratory decline. Firstly, physicians must obtain a complete patient history that will note the course and progression of symptoms and reveal any preexisting comorbidities that could predispose patients to a more severe form of infection. The physical examination is important in guiding the physician toward the lungs as the primary source of infection, after which both imaging studies and microbiological evaluation are necessary. Plain radiography is a valuable tool for visualizing the changes in the lung parenchyma for many infections, including influenza, and bilateral reticular or reticulonodular opacities, as well as patchy areas of consolidation (although other authors suggest that consolidation is absent) are principal features of influenza pneumonia    . Unfortunately, not all patients exhibit typical X-ray findings . Thus, confirmation of influenza virus as the underlying cause through microbiological testing is crucial. Several tests are available    :