(CNN)-Dr. Paul Spiegel looks back at the years he worked for the United Nations helping to coordinate vaccination campaigns for refugees in war-torn sub-Saharan Africa.

There weren't nearly enough doctors. There often wasn't any power. Sometimes medical staffers had to flee from armed militias.

Border agency says it's doing all it can to stop children from dying from flu. Others aren't so sure
So, he wonders: Why does the US government say it's too difficult to vaccinate migrants as soon as they cross over the border from Mexico?
"If we can do it in places like Darfur and South Sudan, I imagine the US government can do it," he said. "Frankly, it's not so hard and it's not so complicated."