Between 1950 and 1970 it was thought, in medical circles and among public health officials in industrialized countries that infectious diseases would inevitably be wiped out. This optimism was also endemic in the Geneva offices of the World Health Organization (WHO). Thanks to advances in hygiene, vaccinations, and antibiotics, microorganisms harmful to human health would be consigned to the dusty pages of the histories of medicine and epidemics. Like the smallpox virus, they would someday exist only in reference laboratories transformed into eco-museums for agents of infection. There would be no more typhus, no more typhoid; meningitis would be cured, sanitariums closed; even the pediatric departments in hospitals would be empty because of the decrease in the number of infectious childhood diseases. There would no longer be any reason to earmark substantial research monies for studies on transmissible diseases, or to allocate new funds for hospital units where these illnesses would cease to be seen. Lacking job opportunities and funding, young specialists in infectious diseases would leave the universities to enter other fields of biological science. The science, medicine, technology, and culture of the twentieth century would be able, it was thought, to eradicate the most dangerous microorganisms. People would live without worries in an artificial, comfortable, and reassuring immunity.
AIDS illustrates the problem of the emergence of so-called "new" viruses, even if there are good reasons for thinking that these new viruses are to viruses what the New World is to the world. Since the appearance of AIDS, everyone sees infectious diseases as a potential danger, a real threat. Our perception of "plagues" is coming closer to that of earlier generations. Isn't it amazing that viruses were forgotten for decades in this way? It was as though the worldwide eradication of smallpox was ipso facto proof that an easy victory would be won over all other viruses. People forgot, it seems, how catastrophic a flu epidemic can be. Strangely, the epidemic of Spanish flu that occurred just after World War I received relatively little attention from the media, even though it killed more people than the Great War itself. The epidemics of smallpox, influenza, AIDS, and other epidemics due to viruses raise questions to which there are no easy answers: How did they happen? Why these viruses? Why at this time and not another?
An undeniable threat is posed by apparently new infectious agents and by other agents expert in resisting therapeutic measures or in getting around organisms' immunity (defense systems). Media looking for sensational news regularly put stories about AIDS on their front pages, even though worldwide, malaria kills every year at least as many people as AIDS has since it first appeared. The wealthy countries' information media must not cause us to forget that despite the search for new medicines and testing of vaccines the malaria epidemic continues to develop dramatically, since the parasite is resistant to drugs and the anopheles mosquito is resistant to insecticides. At the end of the twentieth century, two billion people were exposed to malaria, one-half to one billion people were contaminated, and one to two million died from the disease every year, most of them children. And we mustn't forget prions, the fascinating agents responsible for Mad Cow disease, which is now known to be a new variant of Creutzfeldt-Jacob Disease that affects younger people than the “classical” disease did. Prions are not even living beings, only micro-proteins, but they are transmissible--that is, "infectious"--between different species. Prions resist everything, including heat, and they destroy the central nervous system, leading to debilitating diseases that are always fatal.
The 1960s' confidence regarding microbes is no longer appropriate, but this does not mean that the future is necessarily bleak. We should remain vigilant. What can we propose to ward off the attack of a microorganism? We belong to a species whose numbers are constantly increasing in an exponential demographic explosion particularly worrisome because of its consequences on the environment. Taking this demography into account, we must neither surrender to a Malthusian pessimism according to which the arithmetical increase in the food supply is inadequate to provide for the human species' geometrical growth, nor indulge in an Enlightenment optimism that assumes humans will find a solution to every problem. It is recognized that an increase in the number of individuals promotes the diffusion of microbial diseases.
Biological and cultural evolution will continue; what will emerge from them in the event of microbial dangers? Will we see a devastating cataclysm that will expose us to a worldwide holocaust? Will there be repeated episodes of acute attack occurring against a "background noise" of chronic infections? If our planet is a living being, it will be largely a matter of a violent attack from the inside, a kind of self-destruction or cancer against which James Lovelock's favorite organism will have to find the means to maintain its homeostasis. In his The Ages of Gaia, Lovelock writes that the arrival of Homo sapiens modified the nature of the planet. Anthropic modifications are ongoing, and we do not know where they will lead. Man can corrupt the environment, but he can also protect it. The result of this frequently observed duality will depend on measures that are often difficult to decide upon and even more difficult to put into application.
The relationship between humans and microbes is a complex, dynamic phenomenon; it is not linear, and often corresponds to a chaotic and sometimes wholly unpredictable determinism. We may seek in vain to harmonize Gaia's laws with those of chaos; there will always be an element of unpredictability in this complicated relationship.