The Second Law of Thermodynamics predicts the gradual energetic decay of physical closed systems ("entropy"). Arguably, the Universe as a whole is precisely such a system. Locally, though, order is often fighting disorder for dominance. In other words, in localized, open systems, order sometimes tends to increase and, by definition, statistical entropy tends to decrease. Some physical systems increase disorder, either by decaying or by actively spreading disorder onto other systems. Such vectors we call "Entropic Agents".
Conversely, some physical systems increase order or decrease disorder either in themselves or in their environment. We call these vectors "Negentropic Agents". Human Beings are Negentropic Agents gone awry. Now, through its excesses, Mankind is slowly being transformed into an Entropic Agent. Antibiotics, herbicides, insecticides, pollution, deforestation, etc. are all detrimental to the environment and reduce the amount of order in the open system that is Earth.
Nature must balance this shift of allegiance, this deviation from equilibrium, by constraining the number of other Entropic Agents on Earth - or by reducing the numbers of humans. To achieve the latter (which is the path of least resistance and a typical self-regulatory mechanism), Nature causes humans to begin to internalize and assimilate the Entropy that they themselves generate.
This is done through a series of intricate and intertwined mechanisms: The Malthusian Mechanism - Limited resources lead to wars, famine, diseases and to a decrease in the populace (and, thus, in the number of human Entropic Agents). The Assimilative Mechanism - Diseases, old and new, and other phenomena yield negative demographic effects directly related to the entropic actions of humans.
Examples: excessive use of antibiotics leads to drug-resistant strains of pathogens, cancer is caused by pollution, heart ailments are related to modern Western diet, AIDS, avian flu, SARS, and other diseases are a result of hitherto unknown or mutated strains of viruses. The Cognitive Mechanism - Humans limit their own propagation, using "rational", cognitive arguments, devices, and procedures: abortion, birth control, the pill. Thus, combining these three mechanisms, nature controls the damage and disorder that Mankind spreads and restores equilibrium to the terrestrial ecosystem.