<P> The Italian scholar and physician Girolamo Fracastoro proposed in 1546 in his book De Contagione et Contagiosis Morbis that epidemic diseases are caused by transferable seed - like entities (seminaria morbi) that transmit infection by direct or indirect contact, or even without contact over long distances . The diseases were categorised based on how they were transmitted, and how long they could lie dormant . </P> <P> Italian physician Francesco Redi provided early evidence against spontaneous generation . He devised an experiment in 1668 in which he used three jars . He placed a meatloaf and egg in each of the three jars . He had one of the jars open, another one tightly sealed, and the last one covered with gauze . After a few days, he observed that the meatloaf in the open jar was covered by maggots, and the jar covered with gauze had maggots on the surface of the gauze . However, the tightly sealed jar had no maggots inside or outside it . He also noticed that the maggots were found only on surfaces that were accessible by flies . From this he concluded that spontaneous generation is not a plausible theory . </P> <P> Microorganisms are said to have been first directly observed in the 1670s by Anton van Leeuwenhoek, an early pioneer in microbiology . Yet Athanasius Kircher may have done so prior . When Rome was struck by the bubonic plague in 1656, Kircher spent days on end caring for the sick . Searching for a cure, Kircher observed microorganisms under the microscope and invented the germ theory of disease, which he outlined in his Scrutinium pestis physico - medicum (Rome 1658). Building on Leeuwenhoek's work, physician Nicolas Andry argued in 1700 that microorganisms he called "worms" were responsible for smallpox and other diseases . </P> <P> In 1720, Richard Bradley theorised that the plague and' all pestilential distempers' were caused by' poisonous insects', living creatures viewable only with the help of microscopes . </P>

Microorganisms that cause disease are known as quizlet