Cardiovascular disease (CVD) is the leading cause of mortality and morbidity worldwide. Inflammation has been implicated as a key player in the pathogenesis of CVD, including atrial fibrillation (AF) and viral myocarditis (VM). Moreover, Coronavirus-19 disease (COVID-19), which resulted in a global pandemic crisis, was shown in addition to the lungs to induce inflammation and damage in the heart as well. This thesis describes studies of atrial and ventricular inflammation in AF, VM and COVID-19 that were aimed at improving the clinical diagnosis, as well as the understanding of the pathophysiology of those diseases.