Immunisation used for offensive and defensive purposes during the Second World War.

The best defence against natural or intentional biological agents during armed conflict is usually immunisation, as with typhoid fever, but exceptional circumstances are informative. A large iatrogenic epidemic of hepatitis B occurred in 1942 due to contaminated lots of yellow fever (YF) vaccine used in the US military, even though there was no natural risk of infection. YF vaccine was intended to protect against Japanese Army's use of YF as a biowarfare agent, which did not eventuate. Salmonella typhi was used to infect German soldiers in a Paris café during Christmas 1941 in the knowledge that the café staff but not the soldiers were likely to be immunised against typhoid fever. German Army use of the Weil-Felix reaction to eliminate civilians likely to be typhus infected was subverted by Polish medical officers. They immunised civilians with locally produced Proteus antigens to create false-positive Weil-Felix reactions in order to exempt men from forced labour schemes. Immunisation against biowarfare agents has a mixed record, indicating that vaccines rarely cover well for intelligence gaps.