International Journal of Infectious Diseases (2004) 8, 5—12

REVIEW

Infectious diseases in the 21st century: old challenges and new opportunities Francis A. Waldvogel*
Medical Clinic 2, University Hospital, 1211 Geneva 14, Switzerland Received 7 October 2002 ; received in revised form 30 December 2002; accepted 14 January 2003 Corresponding Editor: Jonathan Cohen, Brighton, UK

KEYWORDS
Infectious disease; Greenhouse Effect; Environment; Migration; Bioterrorism; Nanosciences

Summary Infectious diseases are the confrontation of two worlds, the microbial world and the world of human physiology. Although these two worlds are as a whole governed by the same laws of nature, they show substantial differences: the microbiological world is 1000 times older, and was initiated by the development of the archaea, the ‘living organisms of the extreme’: its biomass and its diversity are immense — two to three billion species or 60% of the total biomass of the planet. The number of pathogens that adapted to man, however, is extremely limited — barely 1000. Thus, over billions of years, an evolution of the microbial world took place from ‘early life’, characterized by chemosynthesis, to the ‘modern pathogens’, and entailed a dramatic ‘concentration’ of life conditions and an adaptation towards a narrow range of requirements — those allowing survival in the human body. Within the last two centuries, these two slowly evolving systems, microbial life and human life, were profoundly modified in an unprecedented manner by a third player, human civilization, with its global impact on the environment through physical, chemical, societal, and climatic determinants. An appreciation of the evolution of infectious diseases in the 21st century and of the development of new diagnostic and therapeutic strategies therefore requires a full understanding of these three domains: human physiology, microbiology, and the environment. This review will put major emphasis on the environmental role of civilization on infectious diseases before considering new opportunities to combat them through novel and creative solutions. © 2003 International Society for Infectious Diseases. Published by Elsevier Ltd. All rights reserved.

Impact of human civilization on the environment
The Greenhouse Effect
The distribution of gases in the earth’s atmosphere — highly reactive O2 , H2 , CH4, among others — is far from the balance predicted by physical laws alone.1 As suggested by James E. Lovelock, life

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1201-9712/$30.00 © 2003 International Society for Infectious Diseases. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijid.2003.01.001

6 profoundly affects the earth’s atmosphere, resulting in a close relationship between the atmospheric, geological and biological environments. These physical and biological considerations have their poetical counterpart in the development of the Gaia hypothesis (Goddess of Earth): the earth is alive, the atmosphere is its circulatory system, life optimizes its own environment in a cognisant way and through self-regulatory feedback mechanisms. What scientific evidence do we have that this interrelationship holds true? Climate change, as currently observed through the global warming phenomenon, is exerting a profound influence on life, and therefore on infectious diseases, by changing their geographic distribution, demographic data, as well as vector conditions.2 Climate, in turn is itself deeply influenced by the evolution of the greenhouse gases of the last 200 years. Since 1800, the CO2 concentration of the atmosphere has risen by 30%, whereas the nitrous oxides have increased by 20%, and methane concentration has risen by more than 100%. These changes affect profoundly the reflection and dissipation of solar energy on the earth’s surface. All these effects, summarized under the general term of the greenhouse effect, have been intensified by additional water evaporation. Global warming is estimated at 1 ◦ C per century. Although such gradual warming could be considered to be inconsequential, it is to be noted that the last glacial period with its dramatic consequences was due to a mere 5 ◦ C temperature change. Global warming has already had a marked effect on the planetary water and air cycles, as shown by a few examples:3 • The sea level has already risen by 35 cm since 1900, changing many demographic conditions and the epidemiology of many infectious diseases such as malaria, dengue and cholera • As another example, the El Nino — the southern oscillation phenomenon which moves warm waters and rain to the Pacific coast of South America in a one-in-every-four year cycle has shown some amplification. Thus, the most rapid spread of the 1991 cholera epidemic in Peru along the Pacific coast, was associated with such a coastal warming. The 1994 Bangladesh cholera epidemic also closely followed the sea surface temperature fluctuations • Other examples and threats include the Rift Valley fever epidemic in East Africa 1997—98, and the present dramatic geographic extension of dengue and malaria.

F.A. Waldvogel

Urbanization, the development of megalopolis
In the last 50 years, the world’s landscape has been profoundly modified by a vast and relentless societal movement from the countryside to the cities, as shown by the following numbers. It is considered today that 47% of the world’s population is urbanized. By 2030, it will be 60% or 4.9 billion people. By as early as 2007, urban dwellers will exceed the rural population for the first time in history. This historical trend towards urbanization is particularly marked in the developing world. If in the industrialized countries, the number of megalopolis has been kept at 6 over 50 years, the number will increase from 5 to 27 in countries in development, creating totally new environmental conditions. This rapid urban growth puts a strain on the capacity of local and national governments to come up with adequate provision of water, electricity, sewage and social services. Historical scourges such as tuberculosis, hepatitis and sexually transmitted infections, and new diseases such as AIDS find ideal breeding conditions in this new, radically altered environment. An example of this dramatic situation is related to the problem of access to water: water is not only elementary to human life, it is also the ideal milieu for pathogen development and transmission. Access to drinking water and sanitation systems are still unsolved problems of humanity. It is estimated that by 2010, one thi...

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