So, too, do many of his fans, now that the painful ailment called eye shingles has temporarily knocked the "Late Show" host off the air. That alone is telling, because Letterman is famous for almost never calling in sick.
Symptoms of eye shingles include a blistering rash, painful inflammation, fever and sluggishness. Or as Letterman, his right eye visibly puffy, described the condition on the show after symptoms began a few weeks ago: "I look like somebody gave me a beating. It's either an irritation, inflammation or infection ... For the love of God, does it hurt!"
It turns out Letterman, who's not scheduled to return to work until next week at the earliest, can blame chickenpox for his troubles.
The virus causing chickenpox, varicella zoster, can remain dormant in the body for years, even decades, before reactivating in nerve cells and causing shingles. Stress, medications, illness or aging can trigger shingles, formally known as herpes zoster.
Shingles can lead to serious eye damage and even loss of vision, especially if not treated quickly and properly, says a new Mayo Clinic study. Treating eye shingles with antiviral drugs, Mayo researchers say, can significantly reduce chances of serious consequences that could require surgery or even cause legal blindness in the affected eye.
"Time is of the essence with diagnosis and treatment," says study author Dr. Keith Baratz. "I tell you, if there's any suggestion of shingles, you treat hard and treat early."
The Mayo study, reported in the March issue of the Archives of Ophthalmology, tracked 323 cases of eye shingles in Minnesota between 1976 and 1998. Of these patients, 202 received oral antiviral medications, while 121 went untreated.
Within five years of getting eye shingles, almost 9 percent of those untreated suffered serious eye conditions, compared with about 2 percent of those who received antiviral drugs.
The Mayo researchers defined these serious conditions as vision loss and damage that requires surgery to repair: scarring of the eyelids, which can prevent proper closing of the eyelids, and trichiasis, in which the eyelids and eyelashes turn inward.
Among those who received the treatment, even one day made a big difference.
Researchers found those afflicted with glaucoma, eye inflammation, swelling of the cornea and scarring of the cornea, which can cause blindness, received treatment an average of 4.8 days after getting eye shingles. Those who had none of these conditions got treatment in an average of 3.8 days, the study says.
"These medications are doing more than just making the patient feel better," says Baratz, a Mayo ophthalmologist. "They're reducing the risk of something very serious happening down the road."
And the risk doesn't necessarily go away when the symptoms do.
"Inflammation effects in the eye could linger literally for decades, so patients who run into problems don't know they're going to run into problems when it happens," Baratz says.
Eye shingles patients normally feel pretty sick, he adds. But for the first day or two, symptoms can be subtle ones such as a red, inflamed eye and minor rash, he says, so shingles can be tough to diagnose immediately.
Of the 121 untreated patients in his study, six had loss of vision, compared with one among the patients who received treatment. All vision loss occurred as a result of scarring of the cornea, the study found.
The study's findings underscore the importance of early diagnosis and treatment, says Dr. Richard L. Abbott, a professor of ophthalmology at the University of California, San Francisco.
"Later, we can treat a lot of the problems, but certainly if you can minimize them or eliminate them by early treatment, that is extremely important," Abbott says. "Patients who come down with shingles or symptoms of this need to seek treatment as soon as possible."
Like the Mayo researchers, Abbott stressed that even after symptoms disappear, eye shingles can still cause serious conditions, including glaucoma and even loss of vision.
An estimated 600,000 to 1 million Americans are diagnosed with shingles annually, the National Institutes of Health reports. Of those who get the disease, about 20 percent will suffer eye shingles, which occurs only in one eye, according to Mayo.
As for Letterman, the prognosis looks good, according to a statement by the physician treating him, Dr. Louis J. Aronne of New York Presbyterian Hospital.
"Dave's condition continues to improve, and his overall health is excellent," Aronne said, "but a complete recovery will require some additional time."
Aronne did not return a reporter's phone calls. A spokesman for Letterman, Thomas M. Keaney, would not comment on the shingles or the treatment but did issue a statement saying the talk show host, who is still recovering, would not return to the show this week.