This article will discuss the importance of Phytophthora infestans in relation to the Irish Potato Famine of the mid-19th century. Phytophthora infestans is an organism that infects potatoes as well as tomatoes and is the cause of the potato famine that gripped Ireland in 1845. The invasive nature and complexity of Phytophthora infestans were responsible for the spread and severity of the epidemic which led to mass emigration and death. . Ultimately, Ireland was ill-prepared economically, socially and intellectually to control the Phytophthora infestans infestation in the mid-19th century. Phytophthora infestans, also known as P. infestans, is an oomycete and a host-specific parasite (Shumann and D'Arcy, 2000). P. infestans primarily infects the plant tissues of potatoes and tomatoes, causing the development of irregularly shaped lesions on the leaves of the plant. Potatoes infected with P. infestans develop large black lesions on the potato skin, making it slimy, soft and completely inedible. The damage that P. infestans inflicts on potatoes was a huge problem in Ireland in the mid-19th century, as half the country depended on potatoes for its livelihood (Mintz and McNeil, 2014). Many Irish peasants could not afford to rent several acres of land. Potatoes were therefore the ideal crop, as one acre of potatoes could support a family for a year (Mintz and McNeil, 2014). The arrival of P. infestans eliminated almost half of Ireland's food supply, causing nutritional deficiencies that led to other diseases such as scurvy and death (Mintz and McNeil, 2014). Ireland's dependence on the potato ultimately worsened the Irish famine.P. infestans breeds and spreads easily in a damp, cool environment...... middle of paper ......er of causalities and emigrants suggests countries other than Ireland could never be prepared to the uncontrollability of P. infestans in the mid-19th century. The devastation caused by P. infestans in Ireland in the mid-19th century was too immense for Ireland to cope with. The invasive nature of P. infestans was responsible for massive emigration and deaths. The complexity and aggressiveness of P. infestans suggests that a potato blight of similar magnitude to that of Ireland would cause as much devastation if it occurred in modern society, as research into P. infestans is new and slowly emerging. The potato blight not only illustrates the instability of Ireland in 1845, but also illustrates the importance of a close relationship between science and history, as both fields of study allow for a diverse understanding past environmental, medical and social problems..