text: A systematic review of pathological findings in COVID 19 a pathophysiological timeline and possible mechanisms of disease progression Since the outbreak of the COVID 19 pandemic much has been learned regarding its clinical course prognostic inflammatory markers disease complications and mechanical ventilation strategy Clinically three stages have been identified based on viral infection pulmonary involvement with inflammation and fibrosis Moreover low and high elastance phenotypes can be distinguished in mechanically ventilated patients based on lung mechanics ventilation to perfusion ratio and CT scans these two phenotypes have presumed differences in their underlying pathophysiology Although essential for therapeutic guidance the pathophysiology of COVID 19 is poorly understood Here we systematically reviewed published case reports and case series in order to increase our understanding of COVID 19 pathophysiology by constructing a timeline and correlating histopathological findings with clinical stages of COVID 19 Using PRISMA IPD guidelines 42 articles reporting 198 individual cases were included in our analysis In lung samples n 131 cases we identified three main histological patterns epithelial n 110 85 with reactive epithelial changes and DAD vascular n 76 59 with microvascular damage micro thrombi and acute fibrinous and organizing pneumonia and fibrotic n 28 22 with interstitial fibrosis The epithelial and vascular patterns can present in all stages of symptomatic COVID 19 whereas the fibrotic pattern presents starting at 3 weeks Moreover patients can present with more than one pattern either simultaneously or consecutively These findings are consistent with knowledge regarding clinical patterns of viral infection development of hyperinflammation and hypercoagulability and fibrosis Close collaboration among medical staff is necessary in order to translate this knowledge and classification of pathophysiological mechanisms into clinical stages of disease in individual patients Moreover further research including histopathological studies is warranted in order to develop reliable clinically relevant biomarkers by correlating these pathological findings with laboratory results and radiological findings thus increasing our understanding of COVID 19 and facilitating the move to precision medicine for treating patients 