3. THE PROCESS OF ‘EMERGENCE’
The primary event—the ‘environmental’ event that initiates a new human infection—is a novel physical contact between potential pathogen and human. The infectious agent mostly derives from an animal source, though some derive from the soil. A particular requirement, usually, is that the potential pathogen is a mutated strain that fortuitously (for the microbe) has become better able to enter and survive in the human host.
This contact event may arise naturally. More usually, it appears that such contacts have come about because of some cultural, social, behavioural or technological change on the part of humans.
The subsequent spread of the ‘new’ infectious disease may depend on either environmental or social factors. These include the following:
- (i) demographic characteristics and processes, human mobility, etc.;
- (ii)land use, other environmental changes, encroachment on new environments;
(iii) consumption behaviours (eating, drinking, and, more generally, culinary culture);
- (iv)other behaviours (sexual contacts, IV drug use, hospital procedures, etc.);
- (v) host condition (malnutrition, diabetes, immune status, etc.).
An example of an emerging disease that primarily reflects social–technological change is hepatitis C. This previously unknown hepatitis virus was identified in 1989, and may have been quietly circulating in humans for a very long time. The advent of illicit intravenous drug use and of medical transfusion has allowed the wider spread and the recognition of this virus.
Meanwhile, the resurgence of previously well-estab-lished infectious diseases—such as cholera, malaria, tuberculosis and diphtheria (in 1990s Russia)—has been primarily attributable to changes in social conditions and behaviours. These include poverty, crowding and the weakening of public health infrastructure. Indeed, this is an old, continuing story. Epidemics have often accompanied periods of great social and demographic transition. Examples include the bubonic plague in fourteenth-century Europe, following the privations and poverty of the feudal system under the stress of several decades of miserable weather and crop failures; the scourges of tuberculosis, smallpox and cholera in the squalid crowded cities of Dickens’s nineteenth-century England; and the ravages of the Spanish influenza following the chaos of World War I.
Various environmental influences, both physical and ecological changes, also influence the resurgence of some of these infectious diseases. For example, the World Health Organization has recently estimated that ca.6–7% of malaria in some parts of the world is attributable to the climate change that has occurred during the past quarter of a century (McMichael et al. 2004).