The Irish Potato Famine in 1845. 750,000 died and 2,000,000 emigrated, many the the US, My wifes side of the family traces back to this event.
During the summer of 1845, a "blight of unusual character" devastated Ireland's potato crop, the basic staple in the Irish diet. A few days after potatoes were dug from the ground, they began to turn into a slimy, decaying, blackish "mass of rottenness." Expert panels convened to investigate the blight's cause suggested that it was the result of "static electricity" or the smoke that billowed from railroad locomotives or the "mortiferous vapours" rising from underground volcanoes. In fact, the cause was a fungus that had traveled from Mexico to Ireland.
"Famine fever"--cholera, dysentery, scurvy, typhus, and infestations of lice--soon spread through the Irish countryside. Observers reported seeing children crying with pain and looking "like skeletons, their features sharpened with hunger and their limbs wasted, so that there was little left but bones." Masses of bodies were buried without coffins, a few inches below the soil.
Over the next ten years, more than 750,000 Irish died and another 2 million left their homeland for Great Britain, Canada, and the United States. Within five years, the Irish population was reduced by a quarter.
http://www.digitalhistory.uh.edu/historyonline/irish_potato_famine.cfm