Trifonova, I. et al. 2019. AETIOLOGICAL STRUCTURE OF ACUTE RESPIRATORY TRACT INFECTIONS AMONG CHILDREN YOUNGER THAN 5 YEARS IN BULGARIA. PROBLEMS of Infectious and Parasitic Diseases. 47, 1 (Jun. 2019), 9–15. DOI:https://doi.org/10.58395/pipd.v47i1.11.