Prompt dileptons produced in the high-energy collisions of heavy nuclei carry the information about the dynamics of the process in its various stages. They may also reveal possible modifications of the properties of hadrons inside a dense and hot medium created in the collisions. What concerns the nature of the medium formed in the heavy ion collisions, the most prominent theoretical candidates are the quark-gluon plasma and hadron gas. To find what is really happening in the high-energy collisions it is necessary to compare theoretical predictions based on different scenarios with the experimental data. We concentrate on the theoretical calculations of the dilepton production from the thermalized hadron gas.To check the reliability of our models and to fix their free parameters we investigate the subnuclear processes which are closely related to dilepton production in hadron collisions, e.g., the hadron production in the electron-positron annihilation, where a huge body of data exists.