Plans-Rubió but inﬂuenza virus infection can cause radiological changes similar to those of other causes of pneumonia. Primary viral pneumonia beguines within 24 hours of the onset of febrile illness with a dry cough that later produces bloody sputum accompanied by tachypnea, progressive cyanosis, and respira- tory failure. Patients deteriorate despite antibiotic therapy, and pneumonia is associated with a high mortality rate. Secondary pneumonia is characterized by the appearance of a new fever and productive cough during early convales- cence. Clinical signs of lobar consolidation can be conﬁrmed by radiology. Streptococcus pneumoniae, Staphylococcus au- reus, and Haemophilus inﬂuenzae are the bacterial pathogens most commonly identiﬁed. Patients with secondary bacterial pneumonia respond to antibiotic therapy. A combined viral and bacterial pneumonia can present with any combination of signs and symptoms. The inﬂuenza virus could produce the exacerbations of the underlying COPD due to the primary viral infection and secondary bacterial infections. Exacerbation of the under- lying COPD is characterized by increased breathlessness, which is often accompanied by wheezing, chest tightness, and increased cough and sputum (Connors et al 1996; Anthonisen 2002). Exacerbation can also be accompanied by nonspeciﬁc symptoms, including malaise, insomnia, fatigue, depression, and confusion. The risk of dying from a COPD depends on the development of respiratory acidosis, the presence of sig- niﬁcant comorbidities, and the need for ventilatory support (Anthonisen 1989, 2002). The epidemiology of inﬂuenza has two dominant charac- teristics: 1) potentiality for causing epidemics and pandemics, 2) epidemic-associated excess mortality in high-risk persons. Epidemics and pandemics depend on the genetic variability of inﬂuenza viruses and susceptibility of the population. Inﬂu- enza epidemics involve susceptible children and adolescents, who spread influenza to adults in their homes and in the community. During epidemics, inﬂuenza is associated with a death-to-case ratio of 30% in patients with high-risk conditions (Glezen 1982). In epidemics, a characteristic time relation- ship exists between peak occurrence of inﬂuenza cases and deaths. In the inﬂuenza pandemic of 1957 in the US, respiratory illnesses reached peak 3 weeks before mortality from inﬂuenza and pneumonia (Kilbourne 1987). This interval represents the spread from the young to the high-risk persons and elderly. Impact of inﬂuenza in patients with COPD Inﬂuenza can have a profound social impact, ranging from an increased demand for medical services to disruption of 44 community services and national economy. The impact of inﬂuenza in patients with COPD can be important due to the higher incidence of inﬂuenza-related complications in patients with COPD (Barker and Mullooly 1980; Glezen et al 1987; Izurieta et al 2000; Neuzil, Mellen, et al 2000; Neuzil, Wright, et al 2000). Inﬂuenza is the most common cause of lower respiratory tract infections in developed countries. In the US, 48 mil- lion cases occur each year, being responsible for 3.9 million hospitalizations and 36,000 deaths (NCHS 1998; Thomson 2004). Physician visits and hospitalizations account for the majority of US$3 billion in direct costs. Indirect costs due to costs related to work have been estimated in US$15 billion per year (Harper et al 2005). During inﬂuenza epidemics from 1979–80 to 2000–1, the estimated overall number of inﬂuenza-associated hospitalizations ranged from 54,000 to 400,000 per epidemic period (Simonsen et al 2000). An average of 220,000 inﬂuenza-related excess hospitalizations occurred per year, with 63% of all hospitalizations occurring among persons aged >64 years (Thomson 2004). Estimated rates for inﬂuenza-related cardiopulmonary problems are 0.4–0.6 per 100,000 in persons aged 0–49 years, 7.5 per 100,000 in those aged 50–64 years and 98.3 per 100,000 in those aged >64 years (Harper et al 2005). Inﬂuenza is associated with a death-to-case ratio of 1–2 per 2000 cases. In the US, the number of deaths at- tributed to inﬂuenza-associated cardiopulmonary problems increased from 19,000 in the period 1976–90 to 36,000 in the period 1990–99 (Thomson et al 2003). This increase can be explained by the higher number of inﬂuenza infections due to the inﬂuenza A H3N2 serotype in the period 1990–99 (CDC 2004). During this period, 90% of detected viruses were from the H3N2 serotype, compared with 57% during the period 1976–90 (Thompson et al 2003). During epidemics the total number of deaths from inﬂuenza can be high due to the large number of cases. An epidemic affecting 20% of the population of the US, Canada, and Europe could result in 70,000–140,000 deaths. In developed countries, 5% of persons aged 0–15 years and 5%–10% of adults aged 15 or more years have chronic bronchitis, emphysema, or unremitting asthma, and more than 52 million people suffer from COPD around the world (Hurd 2000; Pauwels et al 2001). In the US, over 1.5 million new cases are diagnosed from bronchitis, emphysema, or COPD each year. In 2001, the total number of persons aged 25 or more years with COPD was 12 million, of whom 2 million had emphysema, 9.2 million had bronchitis, and 0.9 million had both emphysema and chronic bronchitis (NHLBI International Journal of COPD 2007:2(1)