During illness (if we are talking about flu, colds, acute respiratory infections and other similar "evil spirits"), your body spends a significant amount of fluid to humidify the inhaled air. The problem is that the air reaching the lungs has to be almost 100% humid. Otherwise, the viscosity of the sputum is disturbed, and this, in turn, provokes complications: bronchitis, pneumonia, etc.
Therefore, remember that with such viral diseases, the fight against dehydration is almost the main treatment. (IN NO EVENT DO NOT DISSEND THE DOCTOR'S CONSULTATION !!!)
During illness, the body uses a large amount of fluid, which is necessary for normal recovery. For example, at elevated temperatures, sweating increases. Replenishment of water reserves helps to keep the body in good shape and avoid dehydration.