A vaccine to protect humans against anthrax already exists, but since infection is rare, a widespread vaccination program is not practical. To be effective against anthrax, antibiotics must be given before symptoms actually develop, which is little use to those who are infected. An unvaccinated victim of anthrax usually dies within a few days. That makes anthrax contamination a deadly biological weapon. Researchers at Harvard Medical School, however, have made a discovery that makes possible a new preventive vaccine and a treatment. The method involves generating a mutation of a part of the anthrax toxin known as protective antigen.