Phytophthora infestans is an oomycete or water mold, a fungus-like microorganism that causes the serious potato and tomato disease known as late blight or potato blight. Early blight, caused by Alternaria solani, is also often called "potato blight". Late blight was a major culprit in the 1840s European, the 1845–1852 Irish, and the 1846 Highland potato famines. The organism can also infect some other members of the Solanaceae. The pathogen is favored by moist, cool environments: sporulation is optimal at 12–18 °C (54–64 °F) in water-saturated or nearly saturated environments, and zoospore production is favored at temperatures below 15 °C (59 °F). Lesion growth rates are typically optimal at a slightly warmer temperature range of 20 to 24 °C.

The Great Famine, also known as the Great Hunger, the Famine and the Irish Potato Famine, was a period of starvation and disease in Ireland lasting from 1845 to 1852 that constituted a historical social crisis and subsequently had a major impact on Irish society and history as a whole. The most severely affected areas were in the western and southern parts of Ireland—where the Irish language was dominant—and hence the period was contemporaneously known in Irish as an Drochshaol, which literally translates to "the bad life" and loosely translates to "the hard times". The worst year of the famine was 1847, which became known as "Black '47". During the Great Hunger, roughly 1 million people died and more than 1 million more fled the country, causing the country's population to fall by 20–25% between 1841 and 1871. Between 1845 and 1855, at least 2.1 million people left Ireland, primarily on packet ships but also on steamboats and barques—one of the greatest exoduses from a single island in history.