Three-quarters of dromedary camels in Saudi Arabia have been infected with the virus that causes Middle East respiratory syndrome, or MERS, according to the most thorough survey of the animals there. The finding adds to mounting evidence that camels are a source of the deadly infections in humans.

In September 2012, health experts isolated the first human case of MERS coronavirus, which was discovered in Saudi Arabia and is related to the SARS virus. Since then, the World Health Organization has reported 182 cases and 79 deaths while researchers have scrambled to identify a source of the infections. So far, scientists have found signs that camels and bats harbor the virus, which causes severe pneumonia in humans.