The Haunting of Bly Manor revolves around stories of love and loss, which can all be linked back to the Lady in the Lake, Viola Lloyd. On the Netflix series, the Lady of Bly Manor plagues the halls of her former home after being killed at her sister's hands, but it's an awful illness that weakens Viola in the first place. What is "the lung sickness" Viola suffers from, exactly? There's no clear-cut answer, but it's likely tuberculosis — an unforgiving infectious disease.
Bly Manor's second-to-last episode provides a look into Viola's demise. Her happy life with her husband, sister, and young daughter comes to a sudden halt with one cough. As often happens when an onscreen character falls ill, warning bells start to go off when she coughs blood into a handkerchief. A doctor confirms Viola doesn't have the plague, but reveals she has "the lung." Although her condition is never specified, Viola's pulmonary symptoms suggest she's living with "consumption," or tuberculosis.
There's treatment for tuberculosis today, but in the 17th century, Viola is only expected to live for a few months. She's required to keep her distance from her husband and not allowed to see her daughter, for fear of spreading the sickness. Instead, her sister, Perdita, is tasked with caring for her. Their relationship is close at the start, but cruelty and resentment permeate on both sides until Perdita is driven to smother Viola. Thus begins the tragic haunting of Bly Manor and its residents, for centuries to come.