This study presents the first translation from Latin to English of the Linnaean dissertation Mundus invisibilis or The Invisible World, submitted by Johannes Roos in 1769. The dissertation highlights Linnaeus's conviction that infectious diseases could be transmitted by living organisms, too small to be seen. Biographies of Linnaeus often fail to mention that Linnaeus was correct in ascribing the cause of diseases such as measles, smallpox and syphilis to living organisms. The dissertation itself reviews the work of many microscopists, especially on zoophytes and insects, marvelling at the many unexpected discoveries. It then discusses and quotes at length the observations of Münchhausen suggesting that spores from fungi causing plant diseases germinate to produce animalcules, an observation that Linnaeus claimed to have confirmed. The dissertation then draws parallels between these findings and the contagiousness of many human diseases, and urges further studies of this 'invisible world' since, as Roos avers, microscopic organisms may cause more destruction than occurs in all wars.