Dr. Ian Crozier was declared cured after a harrowing bout with the Ebola virus, only to discover that his left eye was full of the virus. Shocked by the discovery two months after recovery, doctors fought to contain his symptoms and save his sight while his immune system fought the infection, possibly helped by experimental drugs.

After an ordeal during which he nearly went blind and his iris temporarily changed color, Dr. Crozier’s vision is back to normal. He was not contagious while his eye was infected, but the episode raised concerns about when the virus is truly gone from a survivor. How did it linger in his eye?

Viral infections of the eye, even from viruses that do not ordinarily target the eye, are not unusual. Herpes simplex, for example, is known to infect eyes. The immune response to herpes in the eye can cause sight-threatening inflammation called uveitis. The eye is separated from the rest of the body by a blood-tissue barrier, and typically deals with infections through a separate, ocular immune system. Sometimes, however, a virus and its antibodies leak into the eye. Uveitis seems to be most common when the infection has already occurred elsewhere in the body, as the immune cells that remember pathogens, memory B-cells, are most responsible for the inflammation.

But how does a virus persist in the eye after it has been eradicated from the rest of the body?

The answer is that the eye defends itself from the rest of the immune system. There are several tissues in the body, such as the eye, the brain, and the testes, where a systemic immune response is altered or muted based on conditions in those tissues. This situation is known as immune privilege.

Here’s how it works: Immune cells produce a protein called fas. When two fas-carrying cells meet, they self-destruct, apparently in order to regulate the immune response. It turns out that immune privileged tissues such as eyes or testes produce fas proteins also, destroying intruding immune cells and providing protection from dangerous side effects of the immune response.

When a virus slips past the blood tissue barrier into the eye, it is now protected from the body’s main immune response and can reproduce unmolested. This is what happened in Dr. Crozier’s case; as his body fought the Ebola, a separate viral reservoir was able to grow in his eye.

Immune privilege evolved to protect critical systems, such as vision, during illness, but every now and then it backfires.