Respiratory diseases in pig production are important are one of the key sources of productivity losses worldwide. While most of the respiratory problems becoming clinically evident in the late growing phase or during finishing, the agents responsible are thought to colonize the upper respiratory tract of pigs at an early age. According to current knowledge, successful control of these conditions requires, among others, early vaccination of piglets and/or immunisation of their respective dams. To justify this approach, prevalence studies were performed in several countries, to determine the prevalence of key respiratory pathogens in pre-weaned piglets, post-weaned piglets, growers and fattening pigs. Out of these, relatively few reports were published on the prevalence of swine influenza virus, Mycoplasma hyopneumoniae, Haemophilus parasuis, Pasteurella multocida etc. in suckling piglets. This thesis is based on such a study in Hungarian closed farrow-to-finish herds.