The thesis includes the latest findings on the effect of probiotics on acute viral infections of the upper respiratory tract, such as cold, sinusitis, pharyngitis, laryngotracheobronchitis, epiglottitis, rhinosinusitis, otitis media, etc. According to the definition of the World Health Organization, probiotics are living microorganisms that, when used in the appropriate dose, provide the host with health benefits. The use of probiotics has long been proposed for more and more indications, but due to the growing interest, availability and diversity of final products, there are concerns that they are accepted without critical evaluation, quality regulatory control, and sufficiently documented scientific evidence for health benefits in certain pathological conditions and probiotic formulations. Probiotics have their own indication and when using them it is important to use a strain and a CFU dose in accordance with clinical studies and the recommendations of representative authorities. Lactobacillus rhamnosus GG (LGG) is one of the most tested probiotic strains in the world whose efficacy and safety have been clinically confirmed and is recommended according to European and world guidelines as a grade of recommendation A in the prevention of antibiotic-induced diarrhea, prevention of colic in infants, treatment of acute diarrhea, and prevention of respiratory infections in children. Clinical trials have shown a significant reduction in the duration and incidence of upper respiratory tract viral infections with use of LGG culture, and more and more trials are investigating the potential benefit of other respiratory indications (asthma, allergic rhinitis, etc.). The effectiveness of LGG in the prevention of respiratory infections has a background in a complex immunomodulatory effect and consequently coordinated immune communication of the intestinal microflora and the mucous membranes of the respiratory system.