Nothing, there were numerous incidents where it was proven to have happened already.
"Both Polish and Russian partisans used biological weapons in sabotage operations against the Germans. In December 1942, for example, the Gestapo discovered a germ warfare arsenal in a four-room Warsaw house used by the Polish underground. They reported to Himmler the discovery of 'three flasks of typhus bacilli, seventeen sealed rubber tubes presumably containing bacteria, and one fountain pen with instructions for use for spreading bacteria.' Twenty pounds of arsenic had also passed through the house. A few days later, Himmler showed Hitler a captured NKVD order instructing Russian partisans to use arsenic to poison German troops. The raid on the warsaw house apparently failed to prevent the Poles from continuing to use germ weapons. The Combined Chiefs of Staff learned from the Polish liaison officer in Washington, Colonel Mitkiewicz, that in the first four months of 1943, 426 German had been poisoned by the Polish underground; that seventy-seven 'poisoned parcels' had been sent to Germany; and that 'a few hundred' Nazis had been assassinated by means of 'typhoid fever microbes and typhoid fever lice.'"
A Higher Form of Killing by Robert Harris and Jeremy Paxman, page 91