Since ancient times, people have believed that diseases stemmed from unseen organisms, or other forces. Even after microbes were visible with a microscope, it took hundreds of years before people accepted that these tiny creatures could cause disease. This germ theory of disease was controversial when it was first developed, although it is the foundation of many aspects of modern medicine and microbiology.
Scientists now know that around half of all human diseases are caused by bacteria. Viruses, fungi and other microbes are responsible for even more illnesses. This understanding came about slowly as microbiology developed into a mature science. Key components of this knowledge were discovered by French chemist Louis Pasteur. He ran a series of experiments that finally showed the connection between microbes and disease.
Microbial Spontaneous Generation
Spontaneous generation — also called abiogenesis — is the belief that organisms can appear from nonliving materials like water, air and dead flesh. In the fourth century BC, the Greek philosopher Aristotle included this process in his list of methods of reproduction, along with sexual and asexual reproduction, and budding. This belief continued to be accepted during the Middle Ages and beyond. For example, people believed that maggots could appear from decaying meat, and snakes could be born from horse hairs left in stagnant water.
Scientists eventually began to question this belief. In 1668, the Italian physician Francisco Redi conducted experiments with covered and uncovered meat to show that maggots came from flies, not the meat itself. Proponents of spontaneous generation, however, were unconvinced. The use of microscopes only strengthened their conviction. Microbes — now visible for the first time — seemed to appear out of nowhere in substances such as chicken broth. Belief in spontaneous generation continued well into the nineteenth century. In 1858, German scientist Rudolph Virchow proposed that all cells must come from other living cells — biogenesis. It was not until Louis Pasteur’s experiments two years later that spontaneous generation was finally proven false.
Louis Pasteur’s Contribution To Microbes
Louis Pasteur, a French chemist in the 1800’s, made many contributions to microbiology. This included a greater understanding of fermentation by microbes; the development of the germ theory of disease; and the creation of a technique for destroying microbes in perishable fluids, such as milk. The last one was named after him — pasteurization. He also ran a series of experiments in 1860 to show that organisms could not appear out of nonliving materials.
In his first experiment, he boiled beef broth in two flasks. He sealed one, and left the other uncovered. A few days later, the broth in the unsealed flask contained microbes, while the sealed one was uncontaminated. He repeated this experiment, except used flasks with an S-shaped neck. Even though these flasks were open to the air, no microbes grew in the broth. The curved neck allowed air into the flask, but it trapped airborne microbes along the way. With these experiments, Pasteur disproved spontaneous generation.
Microbial Germ Theory of Disease
Pasteur’s experiments also strengthened the germ theory of disease. This concept was not new. In the fourth century BC, the Greek philosopher Democritus proposed that organisms too small to be seen could cause disease. In the seventeenth century, however, English physician William Harvey proposed that outbreaks of disease were caused by movement of the planets, or changes inside the Earth. Subsequent scientific discoveries — such as the microscope and vaccination — did little to convince scientists that microbes and disease were connected.
In the 1700’s, vaccination against microbial diseases was developed. Initially, Turkish women collected pus from smallpox lesions, and injected it into the veins of healthy people. This caused a mild form of the disease, but protected the recipients from severe illness. Doctors later refined this technique. Other evidence for the germ theory came as doctors identified the connection between hygiene and disease. However, it took Pasteur’s work — and that of Robert Koch — to finally show that microbes caused specific diseases.
Public Hygiene & Microbes
With major epidemics occurring in Europe throughout the eighteenth and nineteenth centuries, public hygiene emerged as an important concept. Governments noticed a connection between poor living conditions and outbreaks of diseases such as cholera, smallpox and typhoid. In response, they instituted various sanitation measures: garbage and human waste collection, and increased ventilation in buildings. The measures, however, failed to identify the role of microbes in these diseases.
In 1848, a German physician, made a connection between hygiene and the prevention of microbial disease. He noticed that medical students could spread disease from infected dead bodies during autopsies to pregnant women that they later examined. To prevent that, he required students to wash their hands with chlorinated lime water. The number of women that contracted the disease dropped greatly, although not all doctors accepted this new technique. The use of hand washing — along with the sterilization of medical tools — continues to be used in hospitals to prevent the spread of infectious diseases.
Epidemiology & Microbes
Epidemiology is the science of tracking and identifying the sources of illness. Many of these illnesses are caused by microbes, such as AIDS, Lyme disease, flu and smallpox. Epidemiology builds upon the germ theory of disease, and examines how microbial diseases spread among populations. Modern epidemiology began in the mid-1800’s when an English physician identified a contaminated water pump as the cause of a cholera epidemic. Using information that showed when and where people became sick, he was able to identify the source of the microbes.
Epidemiologists rely heavily on data and statistics. This includes information about the disease, the infected people and the microbes that cause the disease. Depending upon the nature of the microbe, they may also look at weather patterns before an outbreak of the disease, as well as bus and train schedules. This provides them with information about how people and the microbes interact and move around. Epidemiologists work with public health officials to develop the best methods for preventing the spread of microbial diseases.