Question

what was the cause of her death?

Answer 1

Found this as well: 

"O Henry" 
William Sydney Porter

Porter was born on September 11, 1862, in Greensboro, North Carolina. His middle name at birth was Sidney; he changed the spelling in 1898. His parents were Dr. Algernon Sidney Porter (1825–1888) and Mary Jane Virginia Swain Porter (1833–1865). They were married April 20, 1858. When William was three, his mother died from tuberculosis, and he and his father moved into the home of his paternal grandmother.

Here is a source for that information: 

http://www.shirleyassociation.com/NewShirleySite/NonMembers/UnitedStates/OHenry.html

Comments

Thanks Susan for Best Answer!

Answer 2

According to the bio on this profile, Mary Jane died from tuberculosis.