The lung is the human organ mainly affected by the severe coronavirus disease (COVID-19) caused by the novel coronavirus SARS-CoV-2. In this pathology, the dynamic lung function and the respiratory mechanics are compromised, leading to the development of the ARDS (acute respiratory distress syndrome). The resulting damage is the progressive reduction of gas exchange and death in the most critical patients. For these reasons, it is important to study and analyze how this virus adversely affects lung dynamics. The main objective of the present paper proposes a modeling method of SARS-CoV-2 virus particles spread in the 23rd generation of lung tree and the mechanical estimation of how a severe stage of COVID-19 characterized by pulmonary fibrosis affects the alveolar sac expansion and hence the breathing capability of the sick person. In this context, the dynamic analysis of the influence of SARS-CoV-2 spread on human lung under real conditions has been shown by means of a numerical approach. Therefore, a multiphase three-dimensional computational fluid dynamics (CFD) study is performed to estimate the COVID-19 virus particles dispersion throughout a simplified model of the 23rd generation of the bronchial tree, at the alveolar region. Then, a fully coupled fluid-structure interaction (FSI) with the mesh morphing technique and solid displacement characteristics are used to obtain and evaluate a realistic wall displacement during the expansion of the alveolar sac. A comparison is made between a healthy and a diseased lung. These phases are studied under cyclic steady-state conditions The novelties of this analysis are: first, the innovative CFD method proposed in order to model the particles spread inside the alveolar region, and second the evaluation of how the presence of Sars-Cov-2 can affect the mechanical properties of the alveolar sac and damage the lung function of a sick person at an advanced stage of infection, such as a person affected by pulmonary fibrosis.