The tingling began in my left pinkie finger.

A few days later it had spread up my left arm and appeared in the right one, too. Soon my legs began tingling and I felt dizzy, a general unsteadiness that never went away.

Puzzled, I entered my symptoms into several medical Web sites. Up came descriptions of the onset of multiple sclerosis. I was terrified.

For weeks the symptoms persisted. An urgent-care doctor did several motor-skills tests, said he didn’t know what I had, but said that it probably wasn’t MS and sent me home. I went to my regular doctor, who examined my reflexes and strength, and ordered blood tests and an MRI.

On a hunch, my doctor called for another test: my Vitamin D level.

I was skeptical. I’m a lifelong milk drinker, eat fortified breakfast cereals and take a multivitamin. Me, deficient in vitamins? Impossible.

Days later he called with the result. A normal level of Vitamin D in the blood is above 32 nanograms per milliliter, my doctor said. Mine was 16.

He prescribed a megadose of Vitamin D. Through more research on the Internet, I learned of an epidemic of Vitamin D deficiency in recent years.

The vitamin exists naturally in only a handful of foods, including eggs, fish oil and butter, but our bodies produce it most effectively by soaking in sunlight.

I was shocked at the irony: For years I’d worn sunscreen religiously, even in shade, to protect my envelope-white skin. Little did I know that in following conventional wisdom and popular medical advice, I was depriving my body of the vitamin that would help protect it from several chronic diseases – including cancer, diabetes and MS.

In July in the New England Journal of Medicine, Boston University researcher Michael Holick estimated that approximately 1 billion people worldwide are Vitamin D deficient. To combat the epidemic, Holick prescribed doubling the recommended dietary allowance of Vitamin D to 800 IU per day. He also advised getting sensible sun exposure.

My MRI came out normal, calming my anxiety. Six weeks after starting Vitamin D supplements, my symptoms were gone and my Vitamin D levels were normal.

But here’s the astounding thing: Although I had nightmares of costly or painful treatments for my mystery illness, the apparent cure was a six-week supply of pills that, with insurance, cost $1.52. I can continue to treat myself with over-the-counter Vitamin D supplements.

My doctor said he wasn’t sure whether my deficiency caused the tingling and dizziness or whether they happened to coincide with its discovery. But he said he often gives the Vitamin D test to patients.

My late grandmother used to announce before her afternoon walk that she was “going out for my Vitamin D.” One of her quaint Southern sayings, I used to think.

Now I realize how wise she was.