Around 5/27/21, the patient developed a fever. She was previously in good health, and she got the second dose of her Pfizer Covid-19 vaccine at the end of April. Her fever lasted from 05/27/21 - 06/01/21. She took Tylenol every day, and finally the fever decreased after 5 days. Friday 6/4/21 fever came back and continued for 9 consecutive weeks. At the end of week four, her fever reached 39 degree celcius; the patient presented to an infectious disease specialist at an outside hospital to evaluate her fever of unknown origin, for which she was hospitalized from 06/23/21 - 06/25/21. During this hospitalization, per the patient, her ferritin was 35,000, Hemoglobin was slightly over 8 (she did not receive any transfusions), and AST/ALT were elevated. Workup was inconclusive and her fever was attributed to cytokine storm. She was started on steroids (50mg prednisone / day). Her fever persisted for the next few weeks, but her lab values began to normalize the week after starting steroids. By week 8, she only had fever one day, to slightly above 38 degrees celsius and her lab values were continuing to normalize and she thought she was improving. However on Week 9, (07/19/21), temperature increased to 39 and stopped responding to NSAIDs; she continued to be febrile on an off until 7/23/21, so she returned to the previous outside hospital because her temperature reached 40.8 C and lasted at least one hour. In the hospital she was told that there was fluid in her lungs and her lungs had a ground glass appearance. She was told that her labs and imaging were suggestive of Covid, but her Covid test was negative x2. On 07/25/21 her prednisone dose was increased from 50mg to methylprednisolone 1000mg. Per the patient, her ferritin again immediately improved after the steroids were increased.   Of note, every time the patient had a fever, she also experienced the following symptoms: SOB, tightness of chest, weakness in legs and arms, dizziness with difficulty walking straight, headache, blurred vision (a few times vision blacked out for a few seconds), and sore muscles throughout body. In the first few weeks of her fever she felt joint pain "everywhere," but that went away. She denies any history of nausea, vomiting, diarrhea, constipation. On 07/23/21, she had a rash that looked like evenly distributed homogenous red dots on her arms, legs, chest and back that she had never had before.   She was transferred to our hospital on 7/27/21 for further workup because there was no hematologist at the outside hospital.  After an extensive workup, she was fount to have hemophagocytic lymphohystiocytosis with no identifiable infectious, rheumatologic, or neoplastic trigger. On 7/27/21, triglycerides were 527, ferritin was 68,212, she had transaminitis, a low fibrinogen of 118. H-score calculated at 93-96% probability of HLH. An axillary lymph node biopsy in June was negative for malignancy.  Babesia, hep panels, HIV negative and entire infectious work up. Pt had positive CMV and EBV PCR on 7/12/21 outside lab. She had negative PCRs in June; repeat CMV PCR on 7/31/21 negative. West Nile, Tularemia, Q fever, Brucella, leptospirosis, fungitell, histoplasma, cryptococcus, anaplasma, ehrlichia, parvo and bartonella labs all negative. Rheum w/up including anti-smith, p-anca, c-anca, SS-A/B, ANA, anticentromere, DsDNA negative. TNF, IL2/4/13/1-beta/8 negative. IL5/10/6 increased, quantitative IgA and IgM wnl. Kappa/lamda light chain ratio 0.62/wnl; Serum neopterin elevated at 9.5, c/w HLH. A Lumbar performed on 8/4/21 was negative for malignant cells; Flow showing 64% lymphocytes consisting of heterogenous T-cells, negative for b-cells. Bone marrow smear for mycobacterial species / AFB was negative. She was started on HLH directed therapy per HLH94 regimen but she was intolerant refractory to etoposide (neutropenia, persistent fevers), so she was started on treatment with emapalumab on 8/20/21, to which she responded and maintains a response (fevers resolved, counts improved, ferritin improved). She continues to follow up as an outpatient as of 9/7/21.