What was the cause of Emily Brontë's death?

The recorded cause of Emily Brontë’s death was tuberculosis. She died at Haworth on 19 December 1848, aged 30, less than a year after the publication of Wuthering Heights.

Emily’s final illness followed the death of her brother Branwell in September 1848. She developed a severe respiratory illness that was described as inflammation of the lungs, and her condition deteriorated rapidly. Modern accounts commonly identify tuberculosis, or an underlying tubercular condition worsened by the infection, as the likely cause.

Emily refused medical treatment for much of her illness, but eventually agreed to see a doctor when she was already critically weak. Her death came only three months after Branwell’s and was followed by Anne Brontë’s death from illness in May 1849. Tuberculosis had already killed Emily’s older sisters Maria and Elizabeth in childhood.

Pneumonia can appear in accounts of Emily’s final symptoms, which causes occasional confusion. It describes the acute lung inflammation, while tuberculosis is the conventional underlying diagnosis and the answer accepted for this question.