Perhaps the most significant epidemic in regards to US immigration, the Irish potato blight initiated the Great Famine during the 1840s, which ravaged the Irish population. The potato had a great social and cultural significance to the Irish and was an agricultural and dietary staple in Irish society. As the blight destroyed potato crops, hundreds of thousands of Irishmen died from starvation or starvation-related diseases (e.g. TB, measles, intestinal parasites etc.), and over one million immigrated to the US, in total almost a quarter of the Irish population. Irish immigrants flooded off of ships in America and were sent to live in cramped, unhealthy tenement housing. This furthered the spread of crowd diseases in urban population centers where immigrants lived in the tenements. Furthermore, nativists stigmatized the Irish for a number of reasons, and this stigma began the alienation of immigrant populations in the 1800s and early 1900s.
The pathogen itself, Phytophthora infestans, most likely originated in North America and was brought to Europe on ships carrying infected potatoes. When it came to Ireland, the blight infected the Irish potato crops and spread through the close proximity which potato plants are typically grown in an agricultural setting. The blight shows genetic consistency to blight potatoes from the Americas, where the infection did not carry the same repercussions that it did for the Irish due to crop diversification. The HERB-I strain, the most likely cause of the Irish blight, has since died out, but it has been replaced with the new US-I strain. Today, plants are treated with chemicals to prevent the spread of blight, but should a new strain mutate—one which could possibly infect other crops—the results on the world’s already strained food supply could be catastrophic.