[1][2] His distinguished work was recognized at the International Leprosy Congress held at Bergen in 1909.

In 1868 Hansen returned to Bergen to study leprosy while working at Lungegård Hospital (Lungegårdshospitalet) with Daniel Cornelius Danielssen, a noted expert.

[6] In 1873, he announced the discovery of Mycobacterium leprae in the tissues of all people with the condition, although he did not identify them as bacteria, and received little support.

[7] In 1879 Hansen gave tissue samples to Albert Neisser, who then successfully stained the bacteria and announced his findings in 1880, claiming to have discovered the disease-causing organism.

Hansen's claim was weakened by his failure to produce a pure microbiological culture in an artificial medium, or to prove that the rod-shaped organisms were infectious.