Scientist believe that a primate version of HIV—called simian immunodeficiency virus (SIV)—is killing the endangered African chimpanzee at a high rate, The Associated Press reports. It had previously been believed that chimps, like other primates in the wild, did not get sick from SIV.

The new discovery has been viewed as a missing link in the evolution of HIV, which may help researchers find a vaccine or cure for HIV.

According to the article, a nine-year study on chimps at Tanzania’s Gombe National Park discovered that the primates with SIV had a death rate 10 to 16 times higher than uninfected chimps. After looking at that particular strain of SIV, scientists found that it was similar to the virus that first infected humans.

While chimpanzees seem to be vulnerable to the virus, most monkeys and apes that have it often show no symptoms or illness. Daniel Douek, MD, of the National Institute of Allergy and Infectious Diseases, believes that an evolutionary adaptation on the cell receptors might have helped monkeys and apes survive SIV.

“From an evolutionary and epidemiological point of view,” Douek said, “these data can be regarded as a ‘missing link’ in the history of the HIV pandemic.”