Amsterdam, Netherlands ? The remnants of lunch were on Wilco Conradi’s rosy cheeks when the 2-year-old grabbed a large ice cream and ran back to the table.

“Let’s see if this tastes OK,” he said, climbing onto his mother’s lap and drawing a giggle.

Wilco Conradi, 2, was born with the so-called Bubble

This summertime outing at the zoo once was unthinkable for the Dutch boy, who lived his first months in a germ-proof plastic enclosure after being born with severe combined immunodeficiency, or SCID. The plastic enclosure protected Wilco from infections that would have killed him.

But the blond, blue-eyed boy underwent a new gene therapy technique that apparently cured his disease and allowed him to leave his plastic bubble. He needs no medication or special treatment and eats a regular diet.

Wilco is among four boys successfully treated for the inherited disease that occurs in about one of every 75,000 births. The disorder, which is carried by women but afflicts only boys, has plagued Wilco’s family for generations, killing an uncles and two cousins.

The illness renders the immune system ineffective against microbes ordinarily harmless to people with normal resistance. The best-known victim was David, Houston’s famous “bubble boy,” who lived in a germ-proof plastic enclosure until his death at age 12 in 1984.

Many afflicted babies are saved by bone marrow transplants, but for the rest of their lives take monthly intravenous infusions of immune globulin, antibodies culled from donated blood.

Wilco was a baby when he received the experimental treatment at the Hopital Necker-Enfants Malades in Paris. After receiving a single injection of genetically modified stem cells, Wilco now has a normal immune system.

In April, the experimental procedure passed its first major test when Wilco got sick. “He had chicken pox and recovered on his own,” said Dr. Nico Wulffraat, an immunologist at the Wilhelmina Children’s Hospital in Utrecht. “Normally, that would have been lethal.”