In Charles Dickens "A Christmas Carol" what illness was Tiny Tim suffering from?

Timothy 'Tiny Tim' Cratchit is a fictional character from the 1843 novella "A Christmas Carol" by Charles Dickens. Although seen only briefly, he is a major character, and serves as an important symbol of the consequences of the protagonist's choices.

When Ebenezer Scrooge is visited by the Ghost of Christmas Present he is shown just how ill the boy really is (the family cannot afford to properly treat him on the salary Scrooge pays Cratchit). When visited by the Ghost of Christmas Yet to Come, Scrooge sees that Tiny Tim has died. This, and several other visions, lead Scrooge to reform his ways. At the end of the story, Dickens makes it explicit that Tiny Tim does not die, and Scrooge becomes a "second father" to him.

Dickens did not explicitly say what Tiny Tim's illness was. However, renal tubular acidosis (type 1), which is a type of kidney failure causing the blood to become acidic, has been proposed as one possibility. Rickets (caused by a lack of vitamin D) has been proposed as another possibility, as it was a not uncommon disease during that time period. Either illness was treatable during Dickens' lifetime, but fatal if not treated, thus following in line with the comment of the Ghost of Christmas Present that Tiny Tim would die "if these shadows remain unaltered by the Future".