Abstract Body (Do not enter title and authors here): A 60-year-old male patient underwent orthotopic heart transplant for non-ischemic cardiomyopathy three years ago. Recently, he has been followed closely in transplant clinic for persistently elevated donor-derived cell-free DNA; endomyocardial biopsy was negative for allograft rejection. Cardiac MRI showed a global increase in T1 signal with normal T2 suggestive of nonspecific myocardial injury. He later presented with fever and diarrhea. Physical examination revealed pale conjunctivae, non-edematous extremities, and no jugular venous distension. Leukocyte count was 10.1 x10³/uL, hemoglobin 14 g/dL, platelets 407 x10³/uL, creatinine 1.2 mg/dL, AST 53 U/L, ALT 50 U/L, total bilirubin 0.7 mg/dL. Infectious disease workup was positive for norovirus, which was managed conservatively. He returned a week later with sudden right-sided weakness, confusion, and recurrent fever. Brain MRI was negative for acute pathology. Leukocyte count was elevated in the cerebral spinal fluid. No other etiology could be identified. Eventually, microbial cell-free DNA test was positive for toxoplasma gondii. Follow-up endomyocardial biopsy with hematoxylin and eosin stain confirmed diagnosis of toxoplasmosis. He was started on sulfadiazine, pyrimethamine, and folinic acid for six weeks. Follow up biopsy one month later was negative for toxoplasma gondii. Toxoplasmosis can be reactivated in immunocompromised individuals. Cardiac involvement is rare but can lead to myocarditis, pericarditis, or arrhythmias. Post-transplant toxoplasmosis is often due to transmission from the allograft rather than reactivation. Endomyocardial biopsy with special staining can help differentiate between infection and allograft rejection. Microbial cell-free DNA testing can also help isolate rare infectious etiologies, especially when other serum markers are inconclusive. This case illustrates: (1) the importance of differential diagnosis in post-transplant patients and the benefit of microbial cell-free DNA testing, (2) the need to consider various diagnoses in cases of elevated donor-derived cell-free DNA, (3) the necessity for increased awareness of Toxoplasma gondii infection in transplant patients with compatible symptoms, and (4) the crucial role of a multidisciplinary approach in delivering high-quality care in complex cases.