The vast majority of the sick birds brought to clinics and rehabilitation centers have died. At City Wildlife, all birds arrived with symptoms so severe that they died within days or their disease was so advanced that they had to be euthanized. “Not being able to do anything other than alleviate the suffering of these birds has been an emotionally stressful experience,” wrote Dr. Chooljian in an email.

Nearby, the Wildlife Center of Virginia was also overrun by sick, unresponsive birds. “I’ve never lost so many birds,” said Jennifer Toussaint, the centre’s senior animal welfare officer. Concerned that the disease could be contagious to other birds, both City Wildlife and the Wildlife Center of Virginia are now recommending steaming the birds for ingestion.

Although the cause of the disease officially remains a mystery, people have put forward a number of theories. On the internet, the most popular culprit is Brood X, the periodic cicada that has spent the past 17 years underground, only to appear in teeming, singing crowds this year around the time avian disease emerged. While many cicadas’ long-awaited bacchanias went smoothly, others have fallen victim to a zombifying white fungus that some have speculated could cause the disease.

For Dr. Chooljian’s eye symptoms initially resembled those of mycoplasma conjunctivitis, also known as house finch eye disease. But the sick birds did not respond to the clinic’s usual treatments for house finch eye disease, which she added does not cause neurological symptoms.

“Who knows at this point,” said Dr. Casey, adding that she doesn’t want to rule anything out, but rather doubts the cicada hypothesis. “In general, more of us are prone to developing bacterial infections,” she added, pointing to the eye lesions that appear to be similar to bacterial conjunctivitis.

According to Dr. Casey may not be there for several weeks. The dead birds, their eyeballs, and their flesh were subjected to a series of molecular tests for fungi, parasites, and bacteria. However, there is still a possibility that all of these tests will be inconclusive, she said, adding that the diagnostic lab will be holding a handful of bodies for retrospective testing in the years to come.