Answer: Bacteria

Historically it was believed, very strongly, that ulcers were caused by stress. High stress job? Worried about an ill family member? Lost your nest egg in a down-turned stock market? All perfectly plausible reasons for you to have a stomach, or peptic, ulcer.

A primary cause of ulcers, however, isn’t stress (although being stressed all the time is bad for your immune system and health in general) nor is it diet (but again, eating rubbish that keeps you in constant gastro-intestinal distress is terrible for you), it’s bacteria. The bacteria Helicobacter pylori is one of the two leading causes of peptic ulcers (the other being non-steroidal anti-inflammatory drugs)..

The idea that bacteria caused ulcers was so controversial, in fact, that the scientists who discovered it were practically ignored. The pair, Dr. Barry J. Marshall and Dr. J. Robin Warren, went to great lengths to prove the link between the bacteria and ulcers with Dr. Marshall famously drinking a beaker of the bacteria to induce ulcers in himself. Although the medical community was very slow to accept their results (it took decades for the bacteria-as-ulcer-causer-model to take hold), in 2005 the duo won a Nobel Prize in Medicine.

Of even greater importance than the pair of researchers isolating the cause of ulcers was the shift in thinking they helped usher in. Once the link was firmly established between an infectious agent and a chronic disease, the global research community as a whole became more open to investigating possible links between infections and chronic diseases, the effect of the bacterial flora of the gut on health, and the importance of maintaining a healthy microbial environment within the body.