Bacterial Infections/epidemiology; Common Cold/epidemiology; Common Cold/etiology; Picornaviridae Infections/epidemiology; Rhinovirus/isolation & purification; Virus Diseases/epidemiology; Adenovirus Infections, Human/diagnosis; Adenovirus Infections, Human/epidemiology; Adult; Antibodies, Bacterial/analysis; Antibodies, Bacterial/isolation & purification; Antibodies, Viral/analysis; Antibodies, Viral/isolation & purification; Antigens, Viral/isolation & purification; Bacterial Infections/diagnosis; Chlamydia Infections/diagnosis; Chlamydia Infections/epidemiology; Common Cold/diagnosis; Coronaviridae Infections/diagnosis; Coronaviridae Infections/epidemiology; Enterovirus Infections/diagnosis; Enterovirus Infections/epidemiology; Female; Haemophilus Infections/diagnosis; Haemophilus Infections/epidemiology; Humans; Influenza, Human/diagnosis; Influenza, Human/epidemiology; Male; Mycoplasma Infections/diagnosis; Mycoplasma Infections/epidemiology; Picornaviridae Infections/diagnosis; Pneumococcal Infections/diagnosis; Pneumococcal Infections/epidemiology; Polymerase Chain Reaction; RNA, Viral/genetics; RNA, Viral/isolation & purification; Respiratory Syncytial Virus Infections/diagnosis; Respiratory Syncytial Virus Infections/epidemiology; Respirovirus Infections/diagnosis; Respirovirus Infections/epidemiology; Rhinovirus/genetics; Seroepidemiologic Studies; Virus Diseases/diagnosis