During another visit to Shanxi, they had their pictures taken with two Chinese men, standing in roughly the same positions as they did more than half a century ago, when someone - presumably, Ernest Wampler - had pressed the shutter on the four little friends heavily dressed against the forbidding winter of North China. All had grown in those intervening years, especially Eugene Wampler, who stood at over 6 feet tall.

The cave, as with the rest of China, would soon look different. While amazed by the country's development, Eugene Wampler lamented the decline of the cave dwelling in the wake of China's rapid urbanization, and how the area bore little resemblance to the place he once knew.

"We remembered life there as a family among ourselves," says Eugene Wampler, who has filled his own house in Pennsylvania with mementos of his connections with China.

On one shelf, lies the book China Suffers, which was dedicated in part to the 13 Chinese killed by the Japanese in 1940. Ernest Wampler died in 1978, followed by his wife in 1984, both at the age of 93.

Over the past year, Eugene Wampler has been fighting cancer and the disease has noticeably weakened him. But at every mention of a familiar name, or place, or food from China, he rises slightly from a chair, and his face lights up with a broad smile.

"What my mother had started as a nurse - helping women with difficult births - I was finishing with my role in the group looking for a solution for healthy babies in China," he reflects.

Zhou says the introduction of the vaccine meant that over the following 25 years, an estimated 80 million to 120 million cases of hepatitis B infections were prevented in China, as well as an estimated 2 to 3 million cases of HB-related deaths.

During their visit to Shanxi in 2003, the Wampler brothers encountered an old man playing checkers by the roadside. The brothers mentioned tentatively that they lived there some 70 years ago. The man stopped playing, looked at Eugene Wampler intently and said: "Wang Mu Shi."

"That means Pastor Wang. And that was my father," Eugene Wampler says.