H-142 was a hybrid bacterium, bred from Anthrax and Streptococcus, that was created by Dr. Alphonse Moreau. H-142 was among the "H-140 series" that served as part of Britain's biological weapon experiments, in which Moreau's expertise with the H-140 series was of great interest and highly valued by the British government.
H-142 was finally used by the government at the height of the Martian invasion in 1898, when it was delivered by artillery fire on the gathered Martians in South London, effectively causing their horrific demise. But H-142's delivery also affected any unfortunate human beings that were still alive in the area near the alien invaders. The government would later declare them to have been killed by the Martians. This involvement with germ warfare, of which he was not told until after it was done, was the cause of Captain Nemo's leaving the League.