Leptospirosis, a zoonotic disease that causes tens of millions of human cases worldwide each year, is endemic in tropical countries in areas where open sewers or agricultural practices lead to the contamination of water with animal urine. But it has not been clear why city dwellers are more likely to die from the disease than their rural neighbors.
Using molecular techniques to rapidly analyze samples from gutters, puddles, and other surface waters from an urban slum and a rural village in northern Peru, researchers from Peru and the United States found that the urban dwellers were exposed to higher concentrations of more pathogenic species of Leptospira bacteria (Ganoza CA et al. PLoS Med. doi:10.1371/journal.pmed.0030308 [published online ahead of issue August, 22, 2006]). They also found that urban patients were most often infected with the most virulent type of Leptospira; patients from rural areas were usually infected with other Leptospira species.
Stephenson J. Leptospirosis Clues. JAMA. 2006;296(11):1345. doi:10.1001/jama.296.11.1345-d