Tiny Tim's aliment and the prediction of his early demise are some of the key factors that play into Ebenezer Scrooge changing his heart and becoming a "good man". Though the illness is never named within the text, based on the description of Tim Cratchit's actions and symptoms many scholars believe Tim is afflicted with "Ricket Disease". Rickets is "a disease of childhood caused by a vitamin D deficiency" and was not prevalent in England until the industrial revolution (Porter).  Though the disease was rampant amongst the working class and impoverished, it was not fully-understood unit 1919, when Dr. Mellanby published in "4th Vitamin" a formal definition and the cause of Rickets. Mellanby states that "Rickets is a deficiency disease which develops in consequence of the absence of some accessory food factors and factors. It therefore seems probable that the cause of rickets is a diminished intake of antirachitic factor, which is either fat-soluble factor A, or has a similar distribution to it" (Wheeler). Factor A is another name for vitamin D which none of the working-class Victorians in England were getting due to the smog, poor working conditions, and inability due to workload to play outdoors. Rickets is characterized by the bones growing weaker, skeletal deformities, bow-leggedness, dwarfism, and scoliosis. Tiny Tim's weak body, lame leg, and living conditions all suggest he was afflicted with Rickets. 

Dickens, Charles. "A Christmas Carol." 1843

Porter, Tom. “Pregnancy and Rickets in Victorian Britain: A Dangerous Combination.” News, 2021, https://www.bowdoin.edu/news/2021/04/pregnancy-and-rickets-in-victorian-....

Wheeler, Benjamin J., et al. “A Brief History of Nutritional Rickets.” Frontiers, Frontiers, 31 Oct. 2019, https://www.frontiersin.org/articles/10.3389/fendo.2019.00795/full#B22.