The University of Michigan researchers examined the incidence of the disease American cutaneous leishmaniasis, or ACL, in Costa Rica.

ACL -- characterized by skin lesions caused by an infectious organism carried by sand flies -- most commonly affects workers in forested lowlands, but tourists are increasingly at risk as remote tropical areas become more accessible.

A team led by Professor Luis Fernando Chaves examined Costa Rica's ACL case data from 1996 through 2000.

"Contrary to what was previously believed, the more forest you have, even in a marginal population, the more protected you are against the disease," said Chaves. "When we looked just at factors such as climate and the physical environment, we found no specific patterns with respect to the disease. But when we looked at the social data, we found clear patterns according to marginality."

The scientists found socially marginalized people -- those with lower incomes, literacy and levels of education -- had a higher incidence of the disease.

The study appears in the journal PLoS Neglected Tropical Diseases.