Viral infections passed by humans are causing some wild African chimpanzees to get sick and die. Scientists had suspected it for some time, but now German researchers have proved it.

Using evidence gathered about populations of chimps hit by five different respiratory outbreaks between 1999 and 2006 in the Ivory Coast, the researchers found that tissue samples from all that died tested positive for one of two human respiratory viruses.

“Our demographic analyses of chimpanzees suggest that [the infections] started as soon as people got close enough to chimps to transmit diseases,” Fabian Leendertz of Robert Koch-Institut and Max Planck Institute for Evolutionary Anthropology in Germany said. The study, published last week in the journal Current Biology, examined chimps that lived in protected parks where researchers are active but local people don’t travel. As a result, researchers concluded, the sources of the viruses were most likely the researchers themselves or possibly poachers.

Despite this, Mr. Leendertz said research and ecotourism have had a strong positive effect on the survival of great apes by reducing poaching and giving more “political weight” to apes in protected areas. He said researchers have adopted practices to help minimize the risk of infection by, among other things, maintaining a distance of at least 22 feet, wearing masks, and disinfecting their boots regularly.