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AIDS is found, then most of the intravenous drug users who share hypodermic needles will die—not all, because there is a small percentage of people who are naturally resistant, but almost all. The same is true for homosexual men who have many partners and engage in unsafe sex—but not for those who competently use condoms, those in long-term monogamous relations, and again, not for the small fraction who are naturally immune. Strictly heterosexual couples in long-term monogamous relations reaching back into the early 1980s, or who are vigilant in practicing safe sex, and who do not share needles—and there are many of them—are essentially insulated from AIDS. After the curves for the demographic groups most at risk flatten out, other groups will take their place—at present in America it seems to be young heterosexuals of both sexes, who perhaps find prudence overwhelmed by passion and use unsafe sexual practices. Many of them will die, some will be lucky or naturally immune or abstemious, and they will be replaced by another most-at-risk group—perhaps the next generation of homosexual men. Eventually the exponential curve for all of us together is expected to flatten out, having killed many fewerthan everybody on Earth. (Small comfort for its many victims and their loved ones.)
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    Exponentials are also the central idea behind the world population crisis. For most of the time humans have been on Earth the population was stable, with births and deaths almost perfectly in balance. This is called a “steady state.” After the invention of agriculture—including the planting and harvesting of those grains of wheat the Grand Vizier was hankering for—the human population of this planet began increasing, entering an exponential phase, which is very far from a steady state. Right now the doubling time of the world population is about 40 years. Every 40 years there will be twice as many of us. As the English clergyman Thomas Malthus pointed out in 1798, a population increasing exponentially—Malthus described it as a geometrical progression—will outstrip any conceivable increase in food supply. No Green Revolution, no hydroponics, no making the deserts bloom can beat an exponential population growth.
    There is also no extraterrestrial solution to this problem. Right now there are something like 240,000 more humans being born than dying every day. We are very far from being able to ship 240,000 people into space every day. No settlements in Earth orbit or on the Moon or on other planets can put a perceptible dent in the population explosion. Even if it were possible to ship everybody on Earth off to planets of distant stars on ships that travel faster than light, almost nothing would be changed—all the habitable planets in the Milky Way galaxy would be full up in a millennium or so. Unless we slow our rate of reproduction. Never underestimate an exponential.
    The Earth’s population, as it has grown through time, is shown in the following figure. We are clearly in (or just about toemerge from) a phase of steep exponential growth. But many countries—the United States, Russia, and China, for example—have reached or will soon reach a situation where their population growth has ceased, where they arrive at something close to a steady state. This is also called zero population growth (ZPG). Still, because exponentials are so powerful, if even a small fraction of the human community continues for some time to reproduce exponentially the situation is essentially the same—the world population increases exponentially, even if many nations are at ZPG.

    There is a well-documented worldwide correlation between poverty and high birthrates. In little countries and big countries, capitalist countries and communist countries, Catholic countries and Moslem countries, Western countries and Eastern countries—in almost all these cases, exponential population growth slows down or stops when grinding poverty disappears. This