Is believed to set the stage for the development of gastric ulcers.

The answer is Helicobacter pylori

Helicobacter pylori is a type of infection which occurs when the bacteria Helicobacter pylori  infects the stomach. 

It usually happens in childhood and by some estimates over 50% of the world's population may be infected with this bacteria. 

However, most people do not notice since the bacteria does not always shows its symptoms. 

Some conditions that give rise to the bacteria include, living in populated and congested areas with unclean and unreliable water supply, all the conditions found in developing countries.

Carbon monoxide is usually more dangerous indoors.