7: A probable cause of epidemics
What differentiates a minor outbreak of sickness from a plague is likely less to do with the virulence of the illness itself than to the conditions it finds among the population. Where there is a crowded and malnourished population, disease wreaks greater destruction.
Demographic tables are prevented - births and deaths, with a particular interest in the ratio of one to the other - which demonstrate a pattern: after a period in which there is a significant increase in the population, there follows a period where births decrease and deaths increase.
Malthus considers it probable that this is the consequence of an increase in population outpacing the ability to provide the food and accommodations "necessary to preserve them in health." Observing the numbers, it's seen that the proportion of births:deaths increases sharply in established nations, it's generally followed by a period in which deaths exceed births.
As such, it is reasonable to conclude that a plague among people is not the introduction of a particularly virulent disease - but a disease that under ordinary conditions would have done far les damage, visited upon people who live in crowded conditions and weakened by malnutrition. It is also reasonable to infer that pestilence is a natural adjustment to population.
(EN: There follows a copious among of data from various countries and periods in time, which merely heap more evidence on the thesis.)
A "real and permanent" increase in the population of any country is the increase of its means of subsistence, though there are variances, such that the decrease or increase in production is not seen to have an immediate and proportional impact on the population.
For example, what is considered "food" is highly subjective. It's noted that in some nations, the lower classes of people will consume "putrid offal that European laborers would rather starve than eat," and that in some nations, people seem to get by with the smallest possible quantity of food. The converse is also true of a productive nation: people consume more food than is strictly necessary to their sustenance, and food is wasted in times of plenty that would (and will) be used in times of hardship. In America, where there is considerable abundance, even the lowest classes could retrench "very seriously" in a year of scarcity without materially distressing themselves - a famine therefore seems almost impossible. In China and India, where the population is burgeoning and resources are scarce, famine is as regular as the change of seasons.
Even though there is some variance in practice, there is a hard and fast limit to the minimum amount of food a person requires to sustain themselves in good health. So while certain peoples customarily make do with more or less food, and those that have more can more readily retrench, there is a definite bottom beneath which misery, famine, pestilence, and death will surely ensue in all nations.
Another general observation is that the nature of food itself seems to have an impact on how far a given quantity may be stretched. Nations where people consume grain are more populous than those who do not consume grain; and among nations that consume grain, those that consume rice are more populous than those that consume wheat. In England itself, where potatoes have become the staple food, it's likely the population could be better sustained if the cultivation were switched to wheat.
(EN: Malthus doesn't consider the factors driving this phenomenon, though they seem self-evident: grains are a stronger source of carbohydrates than meat or other vegetables, and they are easier to store over long periods of time, to preserve a population through periods of famine.)
Malthus comments that "nothing is so common" as for the ruler of a state to give encouragements to increase the population, hence the power of his dominion - to swell the ranks of the army, to provide plentiful labor for manufacturing goods for export, etc. However, such encouragements are seldom accompanied by any encouragement of agriculture, or any thought of how the increased population will be fed. If any state wishes to increase its population, it should increase the production of food and tend to the condition of its laborers, and the increase will follow. To demand an increase without providing for its sustenance cannot succeed, except to increase famine and death within the realm.
As an example, a common clause in poor-bills is to increase monies paid to the poor proportional to the size of their families. Pitt's bill, specifically, mandates an extra shilling in a laborer's wages for every child in his household above three. The intention may well have been to provide additional funds for those with larger families, but the consequence has been a bounty on reproduction, and a greater scarcity of goods: the laborer is just as productive if he has two children or eight - and while he is given more money, there is no more food to be had, which results in an increase in prices, making food more difficult for all to obtain.
In all, the happiness of a country doesn't depend on its wealth or population alone, but upon its production relative to its population. In a country where the yearly increase in food outpaces the yearly increase in production, people feel prosperous.
That is, a thickly populated or thinly populated country is not happy if there is concern about having sufficient food, nor is a highly productive country happy if all they produce is (or is expected to be) insufficient for the needs of its people. It is only when the inhabitants, however few or many, produce enough for their needs and have reason to believe they will continue to do so, that people are free of concern about their ability to meet their most basic needs.
Pestilence, caused by famine, is the last recourse of nature to adjust a population to its resources. Mankind itself behaves in various ways that contribute to depopulation - but should tyranny and war fail to sufficiently reduce the population, famine and plague will "advance in terrific array" to thin the ranks by tens of thousands.