Bubonic plague, smallpox, rabies or syphilis are some of the diseases for which no cure would have been found without the microscope.In some cases, directly, such as when the German Robert Koch (1843-1910) was able to identify Mycobacterium tuberculosis for the first time, which was behind most of the cases of tuberculosis in the world.The Swiss Alexandre Yersin (1863-1943) did the same with Yersinia pestis, the cause of the bubonic plague.On the other hand, it is true that Louis Pasteur (1822-1895) did not need to see any of the Rhabdoviridae viruses to find a vaccine against rabies.Even so, in an indirect way, the microscope was there, key in the formulation of the germ theory of disease.That is, in the discovery that microorganisms (viruses, bacteria, fungi...) are the cause of a wide range of conditions.If in the 16th century the Italian doctor Girolamo Fracastoro (1478-1553) already suspected that some kind of 'living agents' were behind the ailments, Pasteur could see it with his own eyes.He wouldn't have done it with the naked eye.According to the Pasteur Institute, the human being is capable, at most, of noticing an object of one millimeter at a distance of three meters.Although not bad at all, it is insufficient to see a bacterium, whose size ranges between 0.5 and 5 micrometers in length.Very small, considering that a micrometer is one thousandth of a millimeter.And what about viruses, about 100 times smaller (there are smaller ones), and for which optical microscopes fall short.For someone to be able to see Pasteur's Rhabdoviridae, we had to wait until 1962 and the invention of the electron microscope.However, what is captivating about this story is not the heart-stopping numbers that today's computers record, with millions of increases.The true Rubicon was crossed when scientists discovered the world of the micro: close, but hitherto invisible.This does not mean that the optical properties of lenses, and even of water, were not known as far back as classical Greece.In fact, in the thirteenth century, glasses, magnifying glasses and glasses of all kinds were already instruments of common use.Otherwise, the Dutch mathematician Willebrord Snel van Royen (1580-1626) would not have been able to confirm in 1621 that light rays do not travel at the same speed in air as they do through water or glass.To notice this phenomenon, it is enough to observe the form that a pencil submerged in a glass of water seems to take.It is the principle of refraction, the same one that was used for the first time in the 17th century to deform light rays through lenses to enlarge the image of an object.It was the first microscope.In its simplest format, it was an objective (optic) that was placed as close as possible to the object to be studied, in order to lose as little clarity as possible.Said light traveled through the interior of a cylinder, at the end of which there was an eyepiece through which the observer saw the enlarged image.But who invented it?The truth is that historians are still lost in a slew of candidates, including Galileo Galilei (1564-1642).However, it is not his name that is repeated the most, but that of Zacharias Janssen (c. 1583-c. 1638).Itinerant salesman by profession, more than an inventor, this Dutchman must be considered a survivor.In 1618 he had to flee Middelburg after being caught counterfeiting coins, and it did not last long at his new residence, whence he had to flee again for the same crime a couple of years later.Among so much running, it seems that he also had time to put together a microscope with nine magnifications, that is, one that magnified the size of any object nine times.At least, that is what his son told Willem Boreel (1591-1668), a Dutch diplomat who was leading an investigation to discover the legitimate creator of the device, in 1655.In turn, Janssen Jr. took advantage of the visit to denounce that Hans Lippershey (c. 1570-1619) had stolen the idea from his father.Another one?Indeed.And, for Boreel, one more piece in an investigation he'd gotten off to a bad start.And it is that, already in 1619, during a visit to London, an engineer named Cornelius Drebbel (1572-1633) had shown him a microscope of his own manufacture.With this device, the only thing that can be cleared up is who named it.It was Giovanni Faber (1574-1629), a member of the Italian National Academy of the Lynx.From the Greek micros (small) and scopéo (to look), he came up with it after seeing one in the possession of another lynx colleague, Galileo.Nothing more.In this case, however, perhaps the identity of the inventor matters little.The ones who really turned the microscope into a landmark were the British, Italian and Dutch naturalists who, starting in 1660, began using it to observe nature up close.The Englishman Robert Hooke (1635-1703) did it with almost everything he could.Insects, animal tissues, plants, fossils or mold passed through the platen of it.Later, he collected his research in Micrographia (1665), the first publication to contain detailed illustrations of microscopic observations.Among other things, he described for the first time the structure of snow, ice or crystals present in urine.Although the best known of that book is its use of the term cell, the first.He coined it after observing the dead cells of a cork sheet.From the Latin cellulae, that is, cells, he gave them that name because they reminded him of those of a beehive.Although what was achieved by the Dutch Anton van Leeuwenhoek (1632-1723) is more spectacular.After making a 200x microscope with his own hands, this self-taught man found himself observing almost everything.He was the first to describe protozoa, bacteria, sperm, or red blood cells.As groundbreaking, his descriptions aroused quite a few misgivings in the Royal Society of London, even more so coming from someone without university studies.Perhaps therein lay the key for him, since he was not affected by the prejudices of his medical colleagues, educated in some premises that later turned out to be false.Above all, that ancient belief that pathogens arise spontaneously from organic and inorganic matter (hypothesis of spontaneous generation), or that evils come from fetid emanations from the soil (miasmatic theory of disease).Older than these was the theory of humors, which since ancient Greece dictated that the disease came from an imbalance between the humors of the body (phlegm, blood, black bile and yellow bile).Although these theories had dominated medicine until the 19th century, the discovery of viruses, bacteria, and fungi rendered them all obsolete.Van Leeuwenhoek called them "animalcules," but he was referring to bacteria.The same ones that a century later Pasteur observed and that allowed him to discover the first pathogenic agents.Shortly after, in 1876, Robert Koch was able to show that one of these, Bacillus anthracis, was the cause of anthrax.And so the germ theory of disease was born, changing the history of medicine.But the contributions of the microscope do not end there.In 1931, the Germans Max Knoll (1897-1969) and Ernst Ruska (1906-1988) expanded its possibilities by using an electron beam instead of light to focus the sample.With the electron microscope, they could magnify up to 100,000 times what was observed.It seems like a lot, but today there are already equipment that allow you to see atoms!