What illness did Tanjiro's dad have? In the anime, we have a flashback that mentions Tanjiro's dad who was very ill when Tanjiro was still at a young age.

So, what is the cause of Tanjiro's dad's illness?
Its never stated in the manga why he is so sick.

But if you read the manga you will have a theory


  Tanjiro's dad have The mark, Its stated by upper moon 1 every demon slayer who has the marked will be died before they turn 25 years old. We don't know if Tanjiro's dad have the mark or even he is a demon slayer before, but because he can use breathing technique Dance of the Fire God it's safe to assume he was once a demon slayer. even though it doesn't guarantee if tanjiro's dad ever have The Mark, because not every Demon Slayer have The Mark.