According to Charlie Jamison, Samuel died of Tuberculosis

From a veteran's Administration letter dated Aug 24 1984 to Grace Irene Whale Meneely:

  S. Davis, Private, Company C, 2nd Pennsylvania Heavy Artillery.
  His death occurred on October 1, 1865.  His remains are interred
  in Grave 1041, Section D, Poplar Grove National Cemetery, 
  Petersburg National Battlefield, Petersburg, Virginia 23803.
  This cemetery is under the jurisdiction of the Director,
  National Park Service, Department of the Interior, Washington DC 20240.
  The record further shows that he was originally buried at the Fair Grounds,
  near Petersburg, Virginia.  Perhaps this is the person you are seeking.

In spite of the date of death declared here, Samuel's war record clearly shows that he died on
October 2, 1865 in the St. Petersburg Virginia Fairgrounds Hospital of 'Typho Malarial Fever'.