WORLD SCIENCE FOR MALINOWE HOTELS INTRODUCTION Hydrogen sulphide contained in the therapeutic mineral waters of sulphur-hydrogen sulphide salty waters in Solec Zdrój is a gas necessary for the proper functioning of humans, as evidenced by the fact that it is produced in the vessels, brain, kidneys, liver, pancreas, and in the case of various diseases, deviations from its correct concentration are noted. H2S takes part in memory processes, dilating blood vessels, lowering blood pressure or preventing inflammation of the large intestine and pancreas, preventing inflammatory processes and slowing down metabolism. The only commonly used form of hydrogen sulphide treatment are sulphur-hydrogen sulphide baths, in which hydrogen sulphide diffuses directly through the skin. H2S is mainly synthesized in the human body from cysteine, and its concentration in body fluids and most tissues is 50–300 μmol. In the blood of mammals, including humans, the concentration hydrogen sulphide reaches a level of 30-100 μmol, and in the brain (especially the hippocampus) 50-160 μmol. The most important enzymes responsible for the synthesis of hydrogen sulphide in the body are cystathionine m-synthetase (CBS), mainly in the brain, and cystathionine g-lyase (CSE), which is assigned the main role in the synthesis of this transmitter in the circulatory system. At a pH typical for most body fluids, hydrogen sulphide occurs in an undissociated form in about 30% and, as a lipophilic compound, easily penetrates cell membranes. The most important mechanisms of H2S action include cell hyperpolarization associated with the activation of ATP-dependent potassium channels (KATP) and stimulation of NO synthesis. H2S in the upper limits of physiological concentrations in the cytoplasm of cells usually occurs for a short period of time, causing specific effects, and then it is efficiently utilized. The processing of this compound mainly consists of methylation and oxidation in mitochondria, after which it is mainly excreted in urine. THE ROLE OF HYDROGEN SULFIDE IN RESPIRATORY INFECTIONS There is growing interest in “medical gases” due to their role in infections. Hydrogen sulphide (H2S) is a physiological gaseous mediator that has been recognized as an important signaling molecule in the course of infection. The role of H2S in various infections, such as viruses (paramyxoviruses), bacteria (Mycobacterium tuberculosis), mycoplasmas (Mycoplasma fermentans), and fungi (Aspergillus niger), is the subject of much research because of its therapeutic potential. Since there are no fully effective therapeutic agents, such as antimicrobials or vaccines, against many pathogens, the immunoregulatory and microbiological properties of H2S make it an ideal candidate for the treatment of infectious diseases. H2S has antiviral and immunoregulatory properties. It modulates viral replication and regulates proinflammatory gene expression. Recent studies by Li et al. have uncovered a protective role of H2S against paramyxovirus infections such as human metapneumovirus (hMPV), respiratory syncytial virus (RSV), and Nipah virus (NiV) by modulating viral replication and immune response both in vitro and in vivo. RSV-infected respiratory epithelial cells (AECs) show reduced H2S synthesis via reduced CSE expression and increased H2S degradation. Inhibition of H2S production using a CSE inhibitor in laboratory studies leads to enhanced viral replication and cytokine production. Treatment of RSV-, hMPV-, and NiV-infected mice with an H2S-releasing compound inhibited viral replication and reduced proinflammatory cytokine and chemokine production, suggesting that H2S has a broad inhibitory effect against various paramyxovirus infections. Another laboratory study demonstrated reduced viral replication and improved clinical parameters of lung dysfunction. The protective role of H2S may be due to the reduction of virus-induced cellular infiltrates in the lungs and the subsequent production of proinflammatory mediators. In addition to its anti-inflammatory and antiviral effects, H2S has antioxidant properties that reduce oxidative cell damage caused by viral infection. These results confirm the important modulatory role of H2S in the cellular antioxidant pathway during viral infections. OXYGEN STRESS IS AN INTEGRAL PART OF INFECTIONS OF DIVERSE CLASSES OF VIRUSES Oxidative stress has been implicated in a wide range of etiological factors that cause acute and chronic diseases, such as viral, bacterial, and parasitic infections. In particular, viral and bacterial infections are associated with the induction of ROS/RNS (free oxygen and peroxyl radicals) production, alterations in metabolic pathways, and disease-related complications. In viral infections, the induction of oxidative stress in the host is a prerequisite for the development of infection and long-term viral replication. RNA viruses, such as influenza and paramyxovirus infection, generate ROI (reactive oxygen species) by activating monocytes and leukocytes