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Pertussis is a highly pathogenic respiratory infection that poses a particularly serious threat to unvaccinated children, often leading to severe disease and complications. In adults and adolescents, the condition frequently goes underdiagnosed because its presentation is typically non-specific and can mimic other, more common respiratory illnesses. Although complications affecting the respiratory system are well documented in the literature, cardiac involvement, especially in the form of acute myocarditis, remains a very rare and under-reported condition. Historically, efforts to establish a definitive connection between pertussis and cardiac dysfunction have been limited by the absence of sensitive and specific diagnostic modalities. With advances in cardiac MRI, this case presents an unvaccinated teenager diagnosed with acute myocarditis linked to a pertussis, underscoring the utility of modern imaging modalities and emphasising the importance of vaccination and clinical vigilance in atypical respiratory presentations.