Johannes Fibiger, Danish physician and pathologist, Nobel Prize laureate (d. 1928)
Johannes Andreas Grib Fibiger (23 April 1867 – 30 January 1928) was a Danish physician and professor of anatomical pathology at the University of Copenhagen. He was the recipient of the 1926 Nobel Prize in Physiology or Medicine "for his discovery of the Spiroptera carcinoma". He demonstrated that the roundworm which he called Spiroptera carcinoma (but correctly named Gongylonema neoplasticum) could cause stomach cancer (squamous cell carcinoma) in rats and mice. His experimental results were later proven to be a case of mistaken conclusion. Erling Norrby, who had served as the Permanent Secretary of the Royal Swedish Academy of Sciences and Professor and Chairman of Virology at the Karolinska Institute, declared Fibiger's Nobel Prize as "one of the biggest blunders made by the Karolinska Institute."While working at the Institute of Pathological Anatomy of University of Copenhagen, Fibiger discovered new roundworms in 1907 from wild rats. He suspected that the roundworms were responsible for stomach cancer in those rats. In 1913, he reported that he could experimentally induce cancer in healthy rats using the roundworms. His discovery was considered "the greatest contribution to experimental medicine" at the time. In 1926, he was nominated for the Nobel Prize in Physiology or Medicine along with Katsusaburo Yamagiwa, who had experimentally induced carcinoma by painting crude coal tar on the inner surface of rabbits' ears in 1915. However, they were considered undeserving, and the 1926 prize was not given. In the next year Fibiger alone was retrospectively chosen for the 1926 Nobel Prize.

After his death, independent researches proved that G. neoplasticum cannot cause cancer. Tumours and cancer produced by Fibiger were due to vitamin A deficiency. Historical reassessment of Fibiger's data revealed that he had mistaken non-cancerous tumours for cancerous tumours.

His research method on diphtheria is regarded as the origin of an important research methodology in medicine known as controlled clinical trial.