Hydrogen sulfide is a gas created by decomposing foods.

From Dr. David Klein's Crohns Disease and Colitis Blog:

Undigested protein (chiefly from meat, dairy products, beans, nuts and seeds) will readily putrefy (rot) in our warm gut, just as milk spoils at room temperature on a warm day. Putrefactive byproducts are highly toxic and carcinogenic. They include: methane, hydrogen sulfide and mercaptans (which yield the rotten egg odor when carried out by the methane gas), cadaverine, putrescine, ammonias, indoles, skatoles, leukomaines and host of other toxic and carcinogenic gases and substances. Just as hydrogen sulfide gas will decompose concrete, iron and steel sewer pipes, it will irritate and destroy the flesh inside our intestines and colon.