It is impossible for the human tongue to recount the awful truth… . The victims died almost immediately. They would swell beneath the armpits and in the groin and fall over while talking. Father abandoned child, wife husband, one brother another; for this illness seemed to strike through breath and sight… . In many places … great pits were dug and piled deep with the multitude of dead. And they died by the hundreds, both day and night.6
Agnolo di Tura chronicled this tragic yet common occurrence in Siena, Italy, in 1348. Called the Great Pestilence, the disease was later known as the Black Death or bubonic plague, and it swept through Europe with frightening speed. Spread by the bite of an infected flea, the Yersinia pestis bacteria caused the most devastation recorded in human history. Within four years it killed one-third of the population of Europe, more than 25 million people.
The first symptom to strike a plague victim was a severe headache. The victim would grow weaker and eventually too tired to walk. After about three days the lymph nodes in the victim's armpits and groin swelled to the size of goose eggs. These swellings, called buboes, gave the disease its name—the bubonic plague. The victim's heart would futilely try to pump blood throughout the swollen areas. Blood vessels broke, causing widespread hemorrhaging that blackened the skin. Soon the patient would cough up blood and the nervous system would collapse, causing their limbs to jerk in fits of pain.
Once the lungs became infected, the disease could be transmittable from person to person through the air. At that stage, it was called pneumonic plague, and it swept through villages like wildfire. Within a week the patient was dead.
"I, Agnolo di Tura, buried my five children with my own hands… . And so many died that all believed it was the end of the world."7
The world did not end, but it did change drastically. With more than a third of the population gone, laborers were in high demand. Large tracts of land were suddenly available for those once too poor to own
property. The wealthy became wealthier still, as they accumulated the riches of their dead relatives. Those who survived the epidemic experienced a time of rejuvenation.
They also experienced a time of doubt and inquiry. The methods for dealing with the plague had failed miserably. Many physicians began questioning the validity of the ancient Roman medical philosophies that had been the foundation of all their knowledge. Some began to study the human anatomy and develop new methods of treating the sick. Rather than relying on traditional methods, scientists began proposing new theories of disease and experimenting to prove or disprove their theories. An invisible bacterium capable of killing millions changed history and opened the door to an era known for its startling new ideas—the Renaissance.
Smallpox and the New World
Microbes had an impact on history in the Americas as well. The Caribbean island of Hispaniola had more than a million inhabitants when Christopher Columbus landed there in 1492. Within twenty years, more than a third of the population was dead. Some died at the hands of cruel Spanish masters, others starved to death, but the majority of native islanders died from an epidemic disease they had never seen before—smallpox.
Breathing in the invisible virus particles from an infected person's sneeze or cough spread the smallpox virus from person to person. A week after inhaling these particles, an infected person came down with a high fever, body aches, a headache, and chills. Soon the victim broke out in a flame-red rash that grew fiery, raised, and blistered. These sores or pustules gave the virus its name, variola, derived from the Latin word for spotted. A person who survived might have scars or be permanently blinded. More severe cases that attacked the internal organs resulted in death. This devastating disease spread quickly through a population that had no resistance.
The same thing happened when Hernán Cortés invaded the Aztec city of Tenochtitlán, where he and his soldiers were soundly defeated by the Aztec army. But as the Spaniards fled, they unwittingly left behind a time bomb in the form of a dead Spanish soldier infected with smallpox. Within weeks, the entire capital was under siege by the smallpox virus, which killed one-fourth of the city's inhabitants. According to one Spanish priest, "In many places it happened that everyone in a house died and, as it was impossible to bury the great number of dead, they pulled down the houses over them so that their homes became their tombs."8
The smallpox epidemic spread throughout Mexico and helped the Spaniards defeat the Inca Empire as well. Without the help of the deadly smallpox virus and other epidemics, the Europeans might not have so easily conquered the New World. Smallpox also traveled to Brazil with the Portuguese, killing tens of thousands of Indians there, and marched north to North America with the British, French, and Danish explorers, wiping out scores of Native American villages and entire tribes. The terror was universal. According to one French missionary stationed in Canada, "The contagion increased as autumn advanced; and when winter came … its ravages were appalling. The season of Huron festivity was turned to a season of mourning."9
Other infectious diseases caused by bacteria and viruses may not have had such a profound effect on the world order as the bubonic plague and smallpox, but they also weakened armies, wiped out villages, attacked the poor, and cast blame on those who were different.
Punishment from the Gods
Where could these horrific diseases come from? They mysteriously came upon a person, gripping him or her with terrible symptoms and then quickly spread through a community. Ethnic and religious groups were often blamed for the disease.
Prior to the 1800s most people believed that epidemics like the plague and smallpox were punishments from God. Describing the plague that hit Italy in 1347, the Italian writer Giovanni Boccaccio suggested that the plague signified God's anger at people's wicked way of life. And when the Black Death ravaged England three years later, the archbishop of York said, "This surely must be caused by the sins of men."10
In India, people worshipped Sitala, the goddess of smallpox. Known as the "cool one," she had the power to relieve raging fevers. In paintings and sculptures Sitala is portrayed dressed in red, riding a donkey. She carries a cup of water to cool a victim's wilting thirst and a broom to sweep away the disease. Although people bestowed her with offerings of cool drinks and chilled food, they also feared her, for Sitala could inflict the disease on the undeserving as well. It seemed only reasonable to blame the mysterious illness on a higher power. After all, no one could see another cause.
The earliest written record suggesting that invisible living things might cause illness came from the Roman writer Marcus Terentius Varro. In the first century a.d. he wrote, "Care should be taken where there are swamps in the neighborhood, because certain tiny creatures which cannot be seen by the eyes breed there. These float through the air and enter the body by the mouth and nose and cause serious disease."11
Microbes Come into View
Perhaps Varro was not the only one who suspected that a living organism invisible to the naked eye could exist, let alone cause the deadly destruction that plagued humankind. But it was not a popular thought. There was no evidence that these tiny creatures inhabited the world until a curious amateur scientist named Antoni van Leeuwenhoek saw them for the first time in 1676.
By profession, Leeuwenhoek was a draper (a cloth dealer) who examined threads for flaws with a magnifying glass. His fascination with magnifying lenses and the world they brought into view led him to experiment with single-lens microscopes he made himself. While others used microscopes that enlarged objects only ten times their size, Leeuwenhoek's microscope could magnify up to 270 times. His lenses were so finely made that experts today still are not sure how they were constructed given the technology of the seventeenth century.
What Leeuwenhoek saw under his microscope would open up a new field of science called microbiology. After looking at the matter he picked from between his teeth, Leeuwenhoek recorded for the first time the presence of what are now known as bacteria. He described them as "animacules, very prettily a-moving. The biggest sort had a very strong and swift motion, and shot through the water like a pike does through the water; mostly these were of small numbers."12
Although Leeuwenhoek was the first person to describe bacteria, the scientific community did not take his observations seriously. In 1676 the secretary of the Royal Society in London, wrote to Leeuwenhoek: "Your letter … has been received here with amusement. Your account of myriad 'little animals' seen swimming in rainwater, with the aid of your so-called 'microscope,' caused the members of the society considerable merriment when read at our most recent meeting."13 The members of the Royal Society declined to publish Leeuwenhoek's observations until 1683, when they received more evidence. In the meantime, Leeuwenhoek continued to study pond water, spittle from an old man, insect larvae, and even the spermatozoa in semen. Leeuwenhoek brought the world beneath the microscope into view, but it would take one hundred years before these invisible creatures would be linked with disease.
Putting It All Together
Throughout the 1860s, two scientists, Louis Pasteur of France and Robert Koch of Germany, working independently, collected convincing evidence that infectious diseases were caused by microbes and not by evil spirits or the wrath of God.
Pasteur was a chemist and microbiologist working in France. In his studies of wine making for the wine industry, he learned that microscopic bacteria and yeast organisms caused fermentation, the chemical breakdown of carbohydrates into carbon
dioxide and alcohol. He went on to identify the microorganisms that caused food to spoil and decompose. Before his discovery, people assumed that spoilage was the natural result of chemical breakdown over time. Pasteur found the microbes in milk that caused it to spoil and also devised pasteurization—the process of heating milk to a certain temperature at which harmful bacteria are killed.
After Pasteur's success in the wine industry, the silk manufacturers of France consulted him about the mysterious deaths of their prized silkworms. Pasteur identified two different bacteria that caused the deadly silkworm disease. Pasteur's work provided the world with
convincing evidence that microorganisms cause disease, a concept that became known as the germ theory. Around the same time in Germany, medical doctor and researcher Robert Koch was also putting some of the pieces of the bacterial puzzle together.
Koch was experimenting with ways to grow bacteria in the lab when he developed the process for growing bacteria that is still followed today. By using a gelatinlike substance called agar, which is made from seaweed, rather than blood or tissue from an animal, pure bacterial cultures could be grown without contamination from other blood or tissue cells. Koch's assistant, Julius Petri, created a covered shallow glass dish to hold the agar and the growing culture. Today this commonly used piece of lab equipment bears his name—a petri dish.
Another problem Koch struggled with was making bacteria more visible under a microscope. Some bacteria are very difficult to see, especially if they are mixed with other cells. Through experimentation Koch found that bacteria absorbed a dye made from coal tar, called aniline dye, which made them easier to see under the microscope.
At the time Koch was perfecting his lab techniques, anthrax was a common and debilitating disease that attacked cattle and sheep throughout Europe. Parts of Germany were hard hit by the disease, and Koch set out to isolate the bacterium that caused it. He injected mice with blood taken from the spleens of infected animals and observed how the disease worked as he transferred it from one mouse to another. His study of disease led him to write the criteria that are still used to determine if a microorganism is the cause of a disease. Called Koch's postulates, these criteria state that a pathogenic (disease-causing) organism must be present in every case of the disease. This organism can then be grown, or "cultured," outside the body. An animal inoculated with the culture would develop the same disease. The organism could then be taken from that infected animal and cultured again.
Koch went on to isolate the bacteria that caused tuberculosis and chicken cholera, and Pasteur used Koch's lab methods to expand on his work with anthrax. In order to create a vaccine for sheep, Pasteur weakened the anthrax bacterium by growing it in the lab at higher temperatures than normal. When this weakened bacteria was injected into a healthy animal, it prevented infection from the virulent anthrax bacteria and became an effective vaccine. Pasteur went on to create a vaccine for chicken cholera and rabies.
By the end of the nineteenth century the germ theory was accepted as a scientific principle. Only one problem remained. For some diseases, no microorganisms could be found.
What Could Be Smaller than Bacteria?
Although Pasteur created a vaccine for rabies, he never saw the organism that caused this dreaded disease. Many other scientists who worked on plant and animal infections assumed they were looking for bacteria, but they would never find them. What they did find was something smaller and more puzzling.
In 1886 Adolf Mayer, a German scientist, was researching the tobacco mosaic disease, so called because it left the leaves of the tobacco plant shriveled and mottled. Mayer believed that the disease was caused by a bacterium, but he failed to isolate the elusive organism. In 1892 Russian scientist Dmitri Ivanovski ruled out the possibility that a bacterium caused all the damage to the tobacco plant. He suggested that a smaller pathogen must be at work, possibly a toxin. It was not until six years later that Martinus Beijerinck, a scientist from the Netherlands, showed that the disease was indeed caused by an infectious agent smaller than any other life-form known.
Ivanovski and Beijerinck performed similar experiments. They pressed juice from infected plants through filters so fine that they removed all bacteria. When this filtered liquid was rubbed onto a healthy plant, it caused the leaves to shrivel and discolor. Both scientists discovered that the plant juice could be diluted many, many times and still cause disease. And although they suspected a bacteria-like organism might be at work, it could not be grown separately in a petri dish.
Where Ivanovski and Beijerinck differed was in their conclusions. Beijerinck believed that whatever passed through his filters was some kind of an infective agent
other than bacteria. He did not believe it was simply a toxin, as Ivanovski suggested. Beijerinck filtered and diluted the infective liquid again and again until he was left with such a weak substance that if it were a toxin, it would no longer harm the plant. But when this diluted substance was rubbed onto a healthy tobacco leaf, it shriveled and the disease spread to other parts of the plant. Attempts to grow the organism in the lab failed. Whatever it was, the infective agent would grow and spread only inside plant cells.
In 1898 Beijerinck wrote his conclusions. Using the Latin term for poison, he called the elusive particle a filterable virus. He showed that although it could not be seen, the virus was an infective agent that was not conducive to being cultured in a lab. In his paper he observed, "The contagion, to reproduce itself must be incorporated into the living cytoplasm of the cell into whose multiplication it is, as it were, passively drawn."14
Building on Beijerinck's virus theory, new discoveries were made in rapid succession. That same year, Friedrich Loeffler and Paul Frosch discovered the virus that caused foot-and-mouth disease, which had been killing cattle throughout Europe. They collected pus from the sores of infected cattle and passed it through a filter. They did not find a bacterium, but they did discover that when the so-called filterable virus was injected into a healthy animal, it caused the disease.
It was not until 1900 that a filterable virus was discovered to cause human disease. Yellow fever had been rampant and troublesome throughout Central and South America. It caused almost insurmountable problems for the builders of the Panama Canal. Cuban doctor Carlos Juan Finlay suspected that a mosquito, Aedes aegypti, spread the disease. But this idea did not receive much attention until U.S. Army doctor Major Walter Reed traveled to Cuba and conducted medical experiments. He discovered that the disease was caused by a filterable virus and confirmed that a mosquito was indeed
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the vector (it carried the virus from person to person).
Fifteen years later brought the discovery of a virus that infected bacteria. It was called a bacteriophage (bacteria eater). The definition of a virus was taking shape—an organism that could be passed through the finest filter and still cause an infectious disease in plants, animals, humans, or bacteria. The organism, however, could not be seen and could not be grown in a laboratory. It was not until the 1930s that scientists got their first glimpse of their smallest enemy.
Viruses Come into View
Improvements in microscope manufacturing did not help the search for viruses until a revolutionary machine was invented. In the 1930s German researchers Max Knott and Ernst Ruska created the electron microscope.
Instead of using an ordinary beam of light to illuminate an object, the electron microscope uses electrons, which are accelerated in a vacuum until their wavelength is extremely short. The beams of these fastmoving electrons are then focused on cells. The electrons are absorbed or scattered by the cell's parts and form an image on an electrosensitive photographic plate. This technique allows the microscope to magnify an image up to 1 million times.
For the first time scientists could see the shape of viruses. But the electron microscope still did not reveal what a virus was made of or how it was constructed. That breakthrough came in 1932, when chemist Wendell Stanley used a technique called X-ray crystallography to transform the tobacco mosaic virus into a crystal. This was an amazing feat. Because crystallization is a characteristic of a mineral, a nonliving thing, Stanley's achievement proved that a virus is not a typical living organism. It is essentially a chemical molecule, a protein, with minute bits of genetic material. This discovery won Stanley the Nobel Prize in Chemistry.
The bulk of what was discovered about microbes in the early years of microbiology was through the study of disease and disease-causing bacteria and viruses. One prime goal was finding a way to destroy them.