Technologies don’t have to be cutting edge to make a profound difference in people’s lives. By the early 1990s, diarrheal diseases were killing some 5 million children under the age of five every year. That number is down to about 1.5 million, thanks to oral rehydration salts—a mixture of salt and sugar that can be dissolved in water and administered at home. Zinc is sometimes added to the mix to reduce the severity and duration of diarrhea. This simple innovation has perhaps saved more lives at lower cost than any other.

Vaccines are crucial for public health. But in the developing world, distributing the vaccine to where it’s needed is only part of the problem. How do you administer it in a place where sterile needles might be scarce?...