The lives and scientific achievements of two outstanding Polish biologists - Professors Rudolf Weigl (1883-1957) and Ludwik Hirszfeld (1884-1954) - are presented in the context of the social and political events before and after World War II. The main aim is to recall and emphasise the very modern studies conducted in the two decades between the wars in the Polish scientific centres of Lvov and Warsaw, and the resulting concepts which provided the basis for both the modern microbiological-parasitological experiments and the organisation of post-war teaching and research institutions in Poland. An attempt is made at analysing the effect of scientific paradigms from the boundary of the 19th and 20th centuries on the activity and attitudes of the two outstanding scientists. Their fates coincided in the dramatic war circumstances. Attention is drawn to human and extra-human factors which determined their very different fates in the last, post-war period of their lives. In August 1945 Prof. L. Hirszfeld moved from Lublin to Wrocław where he became famous as the first Dean of the Faculty of Medicine of Wrocław University and the founder of the Institute of Immunology and Experimental Therapy of the Polish Academy of Sciences. At the same time the Weigl Institute in Lvov, world famous for production of the first anti-typhoid vaccine, was never reconstructed in the post-war Poland, and the full scientific potential of the vaccine's inventor remained unrealised in the university circles of Cracow and Poznań, where Weigl was Professsor of biology departments.