Respiratory infections have been reported in Hungary since 1932 on monthly basis to the National Institute of Public Health. These infections were complicated influenza like illnesses, measles, varicella-zoster, scarlet fever, epidemic meningitis, pertussis and diphtheria. The reexamination of these epidemics indicated, that the influenza epidemics were associated with the significant reduction of the number of clinical illnesses caused by other respiratory virus infections. The spread of enteroviruses during summers contributed probably to the decrease of infections caused by the above viruses, too. Epidemics of respiratory illnesses of bacterial etiology were not influenced by influenza epidemics, with the exception of N. meningitidis epidemics, which had been positively influenced by influenza and probably by other viruses (i.e. Respiratory Syncytial Virus). Combinations of microorganisms may significantly increase or reduce virulence or increase pathogenicity of each other. Combination of microorganisms may result in or interfere with emerging infections at the level of microbial physiology.