Who is the greatest person forgotten by history?

Doctor John Snow. 

In the era when the poisonous air measure was considered to be the cause of the disease, Snow was one of the first proponents of germ theory. Snow fought the cholera epidemics that struck London in the 1850s and helped eradicate cholera in the region. 

He also managed to trace the outbreaks of cholera from the water companies, which used the contaminated Thames River as their source of water through the thorough field research and data collections. He proved in this respect to the medical world that the cause of cholera was not toxic air, but contaminated water.