When land is converted from its natural state into agricultural use, the intersection of old diseases with new farming techniques and crops can lead to new outbreaks of infectious disease.
When humans move into previously unsettled areas, and especially if the local ecology is disturbed, new opportunities are created for viruses to cross from animal to human hosts, and then into general populations. Scientists have identified at least 20 completely new diseases in the past 20 years, many of which are believed to have moved into human populations due to the clearing and settlement of new lands.
For example, in Argentina beginning in the 1920s, farmers began planting corn on the pampas, where it was not indigenous. This large-scale cultivation of a newly introduced crop led to a huge increase in the prevalence of a once relatively rare species of mice in the area. The abundant population of mice then exposed farmers to the previously unknown Junin virus, and farmers, in turn, spread the virus to their families and other people, leading to nearly 10,000 deaths over the past 40 years.
Large construction projects have likewise been implicated in the spread of diseases like malaria. The development of irrigation projects, dams, and other construction sites often leads to new bodies of standing water, which create ideal conditions for the proliferation of mosquitoes.
For example, a canal built to irrigate Rajasthan, a very dry region in India, provided a spectacular breeding ground for mosquitoes, which previously existed in small numbers. Naturally, increased transmissions of malaria followed. The introduction of new workers to the area provided the insects with a source of food, and the subsequent migration of these workers to other areas led to additional infections.
An independent review of the project conducted afterward concluded, “The ignition wire of construction-related standing water, and the gunpowder of immigrant labor, [created] an explosion of malaria” (McGinn, Transaction Publishers Society).