This article examines the influence of Lucretius' De rerum natura on the theory of contagious diseases which Girolamo Fracastoro developed during the second decade of the 16th century. It is apparent that the use of the concept of semina morbi was neither an anticipation of modern germ theory, nor a mere adaptation of the terminology of classical atomism. In fact, the combination of the humanist interest in the poem of Lucretius with a renewed attention towards direct observation resulted in the publication in of Fracastoro's De morbo gallico (1530), containing an innovative and effective interpretation of the notion of contagion.