The jab of the needle is a pain, but it protects us from a multitude of microbes. Children in the United States endure as many as 14 vaccine injections by the time they're 16. Adults are immunized to ward off influenza and tetanus; travelers arm themselves against cholera, typhoid and other diseases.

Though injection is a time-honored means of delivering the goods, it has significant drawbacks. Injection equipment can quadruple the cost of a single vaccination. Fear of the needle reduces compliance with vaccination schedules in developed countries. In the developing world, reuse of syringes spreads disease, and lack of refrigeration limits the availability of vaccines. Indeed, the severity of these problems recently prompted the World Health Organization (WHO) to declare war on unsafe injections and to urge the development of oral and nasal vaccines.