The prophylactic use of vaccines has contributed significantly to the decrease in morbidity and mortality associated with many common infections. Diseases such as measles, polio, and tetanus have been largely eliminated from most developed countries, and since about 1950 their incidence has been declining globally because of widespread vaccination programs. However, with the AIDS pandemic and the appearance of antibiotic-resistant bacteria, humankind is once again challenged with developing vaccines to combat both human and veterinary pathogens. Recently a new class of vaccines has been developed, composed not of living organisms or purified proteins as are conventional vaccines, but of nucleic acids.