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This case describes a man in his mid 60s from Mexico who presented with respiratory failure and elevated inflammatory biomarkers. Initial investigations revealed multiple opportunistic infections. A PET-CT scan showed variable uptake in bilateral intraconal masses, dural venous sinuses, paranasal sinuses, lungs, pericardium and both distal femurs. Bone biopsy confirmed clonal histiocytosis (CD68+, CD163+, Fascin+, BRAF-V600E+), leading to a diagnosis of Erdheim–Chester disease (ECD), a rare non-Langerhans histiocytosis affecting multiple systems.As vemurafenib was unavailable in Mexico, pegylated interferon alpha (PEG-IFN-α) was initiated, producing significant clinical and biomarker improvement. The patient was discharged in stable condition and continues close multidisciplinary follow-up with infectious disease, cardiology, nephrology, pulmonology and haematology teams.This case highlights the importance of considering ECD in the differential diagnosis of multisystem diseases of unknown origin, especially with unusual manifestations such as multiple opportunistic infections, and reinforces the value of a coordinated, multidisciplinary approach in managing complex presentations.