We know about Doc Holliday because of tuberculosis. It's also what killed him. John Henry Holliday was born in Georgia, received a good education (Latin, Greek, and dental school), and opened a dental practice in Atlanta. Shortly after that he was diagnosed with tuberculosis (by his uncle, a doctor) and advised that living in a warmer, drier climate would be good for his health.
He moved to Texas and, although he opened a dental practice, soon realized that gambling was a more lucrative and fun profession. He had a hot temper and got into a few gunfights around gambling issues, and this suited him, possibly because of the alcohol he took to control his cough, and possibly because he felt it was better to die of a gunshot than of tuberculosis. He acquired a reputation as someone willing to stand up and use force, and fell in famously with Wyatt Earp and his brothers. Together they moved to Arizona, and made history in the gunfight at the OK Corral in October 1881.
Holliday moved to Colorado in April 1882 and spent the rest of his short life there. In 1887, prematurely grey and sickly, he checked into a hotel with some hot springs hoping the waters would provide relief. They did not. From the website, Outlaws and Gunslingers:
On 8 November 1887, Doc awoke clear-eyed and asked for a drink of whiskey, which he drank with enjoyment. He looked at this bare feet and said, "This is funny." He then drew his last breath from the disease his uncle fourteen years earlier had said would kill him within two years. He had died in the winter, and the ground was frozen solid and covered in ice, preventing the hearse from making it up the steep narrow road to the graveyard on the mountaintop overlooking the town. The well-intentioned citizens of Glenwood Springs laid Doc to rest in an unmarked grave at the foot of the mesa until he could be transferred to the cemetery after the Spring thaws. According to the townsfolk, he never was.
As the years went by, Glenwood Springs grew outward, and today, Doc most likely lies buried in someone’s back yard. Although his grave has been lost, his tombstone rests in the Linwood Cemetery for all to see. It doesn’t even have his full name on it. It says simply "Doc Holliday."
Sources: Wikipedia, Outlaws and Gunslingers