Twelve Diseases that Changed Our World. Irwin W. Sherman

reports of dogs eating dead bodies in the streets. Public works projects were abandoned, and poorhouses were established. The poorhouses (also called workhouses) were mismanaged, overcrowded, and filthy, and the inmates were forced to wear prison-like uniforms. They subsisted on a watery oatmeal. To limit the number of people seeking relief, the Poor Law Extension Act was passed in 1847. This prevented tenant farmers with over a quarter of an acre of land from receiving assistance. In 1847 more than a million died of starvation or diseases such as typhus and cholera; this was a peak emigration year. Indeed, during the next 4 years, 2 million Irish emigrated from Ireland, never to return.

      Conditions were made worse because the British government tried to placate the politically powerful landowners and allowed continued export of food from Ireland while preventing importation of food. The British idea of free trade led to the notion that assistance would weaken the resolve of the Irish peasants. The primary goal of the British was economic: extract the greatest amount of resources and exports from their colonies, thereby benefiting the bankers and landowners. One letter writer to the London Times stated that “Giving more money to Irish relief would be as ineffectual as throwing a sackful of gold into their plentiful bogs.” A Chancellor of the Exchequer said, “Except through purgatory of misery and starvation I cannot see how Ireland is to emerge into a state of anything approaching quiet or prosperity.” In addition, the Irish peasants were so weak from starvation and disease that they could not work the land. Compounding this problem were economic factors: the landowners, in order to meet their losses due to the famine, raised the rents of the tenants, which in turn led to nonpayment, eviction, and destruction of the houses of the tenant farmers. Between 1849 and 1854, at least 500,000 people were evicted. Thousands more were thrown out without official sanction, and homelessness became as much a problem as hunger.

      At first the potato failure was believed to be due to God’s anger over the excesses of the people. Later, it was shown that the failure was due to “late blight,” a disease causing large necrotic areas (called blight) on potato leaves that occurs in the late part of the growing season, e.g., August and September. Late blight reappeared again in 1848 and 1849, and in some places 1849 was as bad as 1847. Many people saw emigration as their only solution. According to the Poor Laws, the landlords were to support the peasants who were sent to the workhouse. This cost £12 a year per person. Some landlords, however, economized and paid for the passage of the peasants to Canada, which cost only £6 a head! The very poor migrated to England—1.5 million went to Liverpool, London, Manchester, and Birmingham—whereas those who were slightly better off and could afford the cost of passage emigrated to the United States. Only about one-fifth of the migrants survived the trip across the Atlantic because of their poor health, the fact that it took weeks to months to cross, and no food was provided on board ship. These were not passenger ships: they were ships ordinarily used for hauling timber and cattle. There was no place to cook and no place to put the sick, and there were no proper latrines. The filth and stench below deck were overwhelming. Many of the passengers carried lice and were infected with typhus. Because of the high death rate on board, they were called “coffin ships.” And it is a bitter irony that in Ireland during this period, while people were starving, grain was still being exported. The potato famine changed the structure of landholding in Ireland—the poorest were evicted, but the landlords were also financially ruined, crushed by the burden of falling income and higher taxation. Many landlords sold out to larger landowners, who in turn were also unpopular with their tenants.

      The Great Hunger’s Cause

      The perils of a single-crop economy have seldom been better illustrated than in Ireland in 1845 to 1849, for at that time without the potato the Irish economy could not survive for very long. While other regions of Europe may have been able to turn to alternative food sources, this was not possible for the Irish. The potato blight was an ecological disaster compounded by the failure of government. Some consider it to be equal to the holocaust. Although theories as to the cause of late blight were many, including an act of God, introduction of the steam locomotive, and excessive uptake of soil water that the potato could not expel, it was the Reverend Miles J. Berkeley who in 1846, after making careful microscopic examination of diseased plants and seeing a whitish felt on the leaf surface (resembling that in moldy bread), proposed that it was none of these but instead was a fungus. Berkeley was mocked, and his contention gained little support. Indeed, the prevailing opinion of the time was that a cold and damp miasma resulted in blight.

      A critical question regarding blight was: which came first, decay followed by fungus or fungus and then decay? In 1861 the great German biologist Anton de Bary clearly showed that late blight was caused by the fungus he named Phytophthora infestans, the “plant destroyer.” To confirm the role of the fungus, de Bary did a simple experiment: he grew healthy potato plants in pots, divided them into two groups, and deliberately dusted spores from the plants with blight onto the moistened leaves of a group of healthy plants; he left the other group (“controls”) alone, making certain that spores could not reach them. Both groups were exposed to a cool, moist environment where the miasma could do its work. In a few days the telltale sign of blight—spots of decay—appeared on the leaves of the fungus-inoculated plants. The control group showed no sign of disease. Clearly, potato plants did not rot because of a miasma or because they took up too much water. de Bary suggested that the microscopic spores ride on the stormy winds and that blight results when rain splashes them on to the leaves. In this way the infection spreads from plant to plant, field to field, and country to country. The significance of de Bary’s work led to a novel understanding of sickness: parasites can be the cause of a disease. Today de Bary and Berkeley are rarely recognized for their pioneering work, yet their experiments anticipated Louis Pasteur’s germ theory of disease by nearly a quarter of a century.

      In late blight, the signs of impending disaster first appear on the leaves in the form of brown-black spots. Under moist conditions the spots enlarge quickly and the plant has a pungent odor. A white fuzzy growth, barely visible, appears in the spots; under the microscope, these are seen to contain the long tube-like threads first seen by Berkeley. The threads (called hyphae) divide and twist like snakes to form an extensive network; they penetrate the plant tissues and act like “soda straws,” allowing the fungus to “drink” the rich nourishing sap of the potato until the leaf and stem are literally sucked dry. This takes only 3 to 5 days. At the tip of each filament a swelling develops; within the swelling, microscopic spores are produced. Millions of spores can be produced on an infected leaf, and each spore is so tiny that 500 of them would be no larger than the period at the end of this sentence. The spores germinate, giving rise to either tube-like threads or swimming spores that can also form threads. The tubes enter the leaf through its microscopic pores (stomata), or the leaf tissue is eroded by digestive enzymes released by the hyphae. The cycle of spore formation and germination is favored by moist, cool conditions. But how does blight on the leaf produce rotten potatoes? de Bary buried healthy potatoes in the soil, shook the spores from blighted leaves on the soil surface, and gently watered them as if it were raining. The spores washed down, and when the potatoes were dug up, they too were blighted. Clearly at harvest time millions of spores are washed from the leaves so that the fleshy tuber itself becomes infected. Its skin is discolored with brown-purple blotches resembling bruises, and as the microscopic threads of the fungus penetrate deeper, the tuber begins to rot. Dry rot of tubers by Phytophthora is followed by wet rot of the potato due to other microbes in the soil.

      One of the mysteries concerning the late-blight fungus was how it was able to survive the cold of winter. Did it overwinter in the soil or in the tuber? Although observations of the hyphae under freezing conditions showed that they were too fragile to withstand low temperatures, within the tuber itself the fungus was protected and able to survive very low temperatures. Since only the tuber was kept through the winter, the blighted potatoes provided the source of infection for the next season’s crop. Because in a single growing season it is possible to get many cycles of late blight, it is considered to be a compound-interest disease with great powers of amplification that can lead to an explosive outbreak. As a result, an entire potato crop can be quickly destroyed. Indeed, one infected tuber per 2.5 acres can cause an epidemic of late blight, especially when the weather is right: cool, with high rainfall and humidity.

      Where did the “plant destroyer” come from in the