276 Table 1 Persons at higher risk for influenza complications Children <2 years Adults ≥65 years Women who are pregnant or postpartum (within 2 weeks after delivery) Individuals with chronic medical conditions Pulmonary disease (including asthma) Cardiovascular disease (except isolated hypertension) Active malignancy Chronic renal insufficiency Chronic liver disease Diabetes mellitus Hemoglobinopathies (including sickle cell disease) Immunosuppression 1 Any neurologic condition that can compromise handling of respiratory secretions 2 Native Americans and Alaska Natives Morbidly obese persons (body-mass index ≥40) Residents of nursing homes and other chronic-care facilities (1) Including HIV infection (particularly if CD4 <200 cells/microL), organ or hematopoietic cell transplantation and inflammatory disorders treated with immunosuppressants (2) Including disorders of the brain, spinal cord, peripheral nerve, and muscle such as cerebral palsy, seizure disorders, stroke, intellectual disability, moderate to severe developmental delay, muscular dystro- phy, or spinal cord injury Adapted from the recommendations of the Advisory Committee on Immunization Practices (ACIP) [40 ] this article, we review the latest available data for the diagnosis of influenza lower respiratory tract infection, as well as for treatment and prevention strategies. Clinical and Radiological Diagnosis Signs and symptoms of upper and/or lower respiratory tract infection, along with systemic involvement in the form of fever, myalgia, and headache, are usually the main present- ing features of the disease. In the context of an outbreak, otherwise healthy subjects presenting with a self-limited acute febrile respiratory illness usually require no further diagnostic procedures. In two retrospective studies that ex- amined which clinical signs and symptoms are most predic- tive of influenza infection in patients with influenza-like illness, cough and fever were the only symptoms signifi- cantly associated with a positive PCR test for influenza [3, 4]. In another study, no isolated symptom or sign was able to accurately predict influenza infection, though the absence of fever, cough and nasal congestion significantly decreased its likelihood [5]. In general, patients diagnosed with pandemic H1N1 influenza A virus had similar signs and symptoms compared to those with seasonal influenza. However, these Curr Infect Dis Rep (2012) 14:275–283 patients had gastrointestinal manifestations more frequently [6, 7], were more likely to have pneumonia [8], and also had higher rates of extrapulmonary complications, intensive care unit admission, and death [9]. Pneumonia is the most frequent and severe complication of influenza, most commonly presenting in high risk patients (Table 1). Primary influenza pneumonia represents direct lung involvement by influenza virus, and should be suspected in non-resolving influenza infections. Typically, primary influ- enza pneumonia presents in chest x-rays with bilateral reticu- lar or reticulonodular opacities. Less frequently, focal areas of consolidation can be seen, particularly in the lower lobes. High-resolution computed tomography may show ground glass opacities with or without multifocal peribronchovascular and subpleural consolidation [10]. The cytopathic effect of the influenza virus on the tracheo- bronchial epithelium may predispose to secondary bacterial pneumonia [11, 12]. Secondary bacterial pneumonia must be suspected whenever there is an exacerbation of fever and respiratory symptoms after initial improvement in a patient diagnosed with acute influenza. Leukocytosis, instead of a normal or low white blood cell count, and lobar consolidation on chest imaging, instead of the diffuse pattern that is typical of viral pneumonia, are also suggestive [13]. In an observational study of 543 hospitalized patients with H1N1 influenza A infection in Spain, 43 % of the 243 patients in which chest radiographs were performed had pneumonia, 83 % of the 210 patients who had microbi- ologic confirmation had primary influenza pneumonia, and the remaining 17 % had concomitant secondary bacterial pneumonia. Bilateral pneumonia occurred in 48.3 % of patients; Streptococcus pneumoniae being the most frequent pathogen [14]. Several reports have identified methicillin- resistant Staphylococcus aureus (MRSA) as the etiologic agent for severe community acquired pneumonia (CAP) in otherwise healthy young patients with influenza [15–17]. In another study that investigated the incidence of community- acquired MRSA pneumonia in H1N1 influenza patients, 50 patients of 4491 (1 %) laboratory-confirmed pandemic in- fluenza A (H1N1) cases had a bacterial respiratory tract pathogen. The most commonly cultured organisms were S. pneumoniae (16 patients), S. aureus (13 patients) and Hae- mophilus influenzae (9 patients); MRSA was detected in only 2 patients [18]. In contrast, among 838 children and adolescents admitted to 35 intensive care units in the U.S. with confirmed or probable severe H1N1 influenza A infec- tion, 48 % of the 71 patients with suspected diagnosis of early S. aureus coinfection had MRSA [19]. Non-seasonal influenza infections have specific clinical man- ifestations. Pneumonia related to the 2009 H1N1 influenza A pandemic was also found in many cases to be rapidly progres- sive, leading to respiratory failure and ARDS [20 , 21 ]. Addi- tionally, the risk for complications and death due to that