What was the real reason behind Tanjiro's father's death ?

We know that if a person is born with a mark and has the ability to use sun breathing, they could continue to live to be 25 years old.

If they did not have it from birth, though, they would pass away before becoming 25. Tanjiro claimed that his father's death was unrelated to the demon slayer mark because his father had a mark from birth.

Though he was not a member of the demon slayer corps, some people believe he died fighting demons. However, this is also incorrect.

His death was caused on by disease, but the fact that he was powerful enough to kill Mountain Beard with merely an ax when he was ill.

When he realized he would die soon and wanted to pass on all he had learnt from his forefathers, he taught Tanjiro at a very young age.