More than 70 bacteria, viruses, parasites, and fungi are serious human pathogens.1 Vaccines are available against some of these agents and are being developed against almost all the other bacteria and viruses and about half of the parasites. Table 1 lists infections for which there are now licensed vaccines and those for which a candidate vaccine has undergone a phase 3 clinical trial,2 indicating that a vaccine will probably be licensed within 5 to 10 years. Traditionally, attenuated vaccines were made by repeated passaging of the infectious agent in tissue culture or animal hosts until its virulence was greatly decreased but its . . .