Acute respiratory disease in Spain: seven years of experience The clinical and epidemiologic features of viral and nonviral pathogens involved in acute respiratory diseases are described in the context of cases of infection (especially atypical pneumonia and bronchiolitis) studied at the Centro Nacional de Microbiologia, Virologia e Immunologia Sanitarias in Madrid during a 7-year period (1979-1986). These etiologies were demonstrated in 1,637 (36.2%) of 4,521 cases. Among viruses, respiratory syncytial virus most frequently infected children; influenza virus showed the same pattern of circulation as in other European countries. Of nonviral agents, Mycoplasma pneumoniae and C. burnetii were most often involved in lower respiratory tract infections, with a variable predominance in patients of different ages. A high proportion of cases of M. pneumoniae infection occurred in infants and children aged less than 1 year, and most of these cases occurred during spring and summer. The majority of Q fever cases, including those observed in two outbreaks, occurred in the northern region. 