From the Richmond Daily Dispatch September 8, 1864:
… Mr. W. B. Egerton, a citizen of Petersburg, died in the Federal prison at Elmira, New York, on the 21st ultimo.
Elmira started accepting Confederate prisoners on July 6, 1864. By August it was already holding thousands.
From a Seneca County, New York newspaper in August 1864:
REBEL PRISONERS. – There are now over eight thousand rebel prisoners in the barracks at Elmira.
I don’t know the specific cause of Sergeant Egerton’s death, but it could certainly have something to do with overcrowding. Chemung County History points out that there were only enough barracks to house 5,000 prisoners. The site also says that the photo up top of the camp is from December 1864, but it would seem that tents were already probably being used in August. According to Wikipedia:
During the 15 months the site was used as a prisoner of war camp more than 12,100 Confederate soldiers were incarcerated there; of these, nearly 25% (2,963) died from a combination of malnutrition, continued exposure to harsh winter weather, and disease from the poor sanitary conditions on Foster’s Pond combined with a lack of medical care. The camp’s dead were prepared for burial and laid to rest by the sexton, an ex-slave named John W. Jones, at what is now Woodlawn National Cemetery.
Records at the Chemung County History site give Sergeant Egerton’s date of death as August 22, 1864. His Woodlawn Cemetery number is 38.