How can a viral infection change the color of a person’s eye? Months after Dr. Ian Crozier thought he had recovered from Ebola, he was stunned to find himself developing intense eye pain and fading vision. The inside of his left eye was still occupied territory, full of live, replicating virus. And one morning during this siege, he looked in the mirror and saw that his iris had changed from blue to green.

Although such color changes are rare, they do occur from time to time during viral infections, eye doctors say. Herpes viruses are the most common cause, but other viruses can do the same thing. The changes in hue are almost always permanent.

But in Dr. Crozier’s case, his eye color went back to normal as he recovered. Why only one eye was affected, and how the changes happened are mysteries. His doctors could not explain them, and two outside experts were also stumped, particularly when the blue-green shift reversed itself.