By Barry C. Fox, M.D., University of Wisconsin
The plague and malaria are two of the oldest infectious diseases that had a significant impact on the history of medicine. However, the concept of germ theory did not exist until the mid-19th century when the microscopic organisms or germs that affected the patients were identified.
The plague was one of the deadliest diseases that killed almost 200 million people worldwide. It was clear that the plague was contagious, but the germs that made it so deadly were not identified until the microscope was invented.
The Invention of the Microscope
With the discovery of the microorganisms, two great figures in medical microbiology, Louis Pasteur of France and Robert Koch of Germany, proved that bacteria caused diseases and isolated specific microbes as the cause for specific diseases.
The microscope was invented five centuries after the plague appeared and only then were scientists and doctors able to identify the microscopic organisms as the culprits that spread the disease.
Over time it was learned that disease outbreaks are sometimes caused by bacteria, viruses, or parasites, and with modern technology, the causes of most infectious diseases can easily be determined.
Learn more about the milestones in infectious disease history.
Germs are frequently transmitted from animals to humans. In fact, over 60 percent of diseases are thought to have origins in animals, like monkeys, birds, bats, and rodents. In the case of Ebola, fruit bats were the most likely culprits.
There is also evidence that diseases such as influenza and HIV started in other species. The major cause of this transmission capacity is the increased human activity in wilderness areas, for example, where rainforests are being cut down.
Plague Germ Identified
During an outbreak of the plague in Hong Kong, Louis Pasteur sent Alexandre Yersin to try to identify the germ. He drew fluid from a dead man’s buboes and studied it under the microscope.
He identified the bacterium, later named after him, called Yersinia pestis. Next, he injected the germ into guinea pigs. The pigs died a few days later and that convinced him that he had identified the plague germ.
He also tested some of the dead street rats and found that they contained the same bacteria.
Transmission of the Plague Germ
Once the plague germ was identified, the next step was to identify how the germ was transmitted. Pasteur sent a French army physician, Paul-Louis Simond, to figure out the vector of transmission.
Simond determined the real culprit for disease transmission was a little flea, which fed on a rodent that carried the bacteria. The flea became the vector of the disease, transmitting it from animals to humans. Thus, rat-infested ports all around the world could have served as the impetus for spreading diseases.
Learn more about how vaccines save lives.
Why Was the Plague Hard to Eliminate?
In the mid-1300s, the plague swept through Europe, Africa, and the Middle East, resulting in the deaths of millions of people. Many people suffered and died due to the lack of knowledge of germ theory or without the tools for diagnosis of illnesses.
The plague could remain alive and be potentially deadly even up to three days later in a dead person. In addition, there are more than 80 species of fleas that can transmit the plague. Fortunately, with control of the flea vectors, and with antibiotics given in a timely manner, today, 85 percent of plague cases are cured.
This is a transcript from the video series An Introduction to Infectious Diseases. Watch it now, on Wondrium.
Global Impact of Malaria
Nearly half a million years old and one of the most infectious diseases, malaria is a parasite transmitted by mosquito bites that affects the functioning of the red blood cells that control the delivery of oxygen to the body. It kills nearly one million humans every year.
While the vast majority of global malaria mortality is in Africa, malaria still frequently occurs in Southeast Asia and the Amazon region of South America.
Malaria has also changed the course of human history. In the early 1900s, the Panama Canal was being built. The wet and hot environment was the perfect breeding ground for malaria and another viral illness called Yellow Fever.
In 1897, the link between the mosquito and malaria disease transmission was proven, and Dr. William Gorgas set out to eradicate the insects by eliminating standing water, pouring oil on immature malaria forms called larvae, and by fumigation of living spaces.
Why Is Malaria Difficult to Eradicate?
The reason malaria is difficult to eradicate is that there is a continuous cycle of infection occurring. First, a human is bitten by an infected mosquito carrying the malaria parasite. It then injects the parasite into the human, causing illness. Simultaneously, an uninfected mosquito can become a new carrier by biting a human who is infected, continuing the cycle of transmission.
Environmental factors like global warming are thought to predict a dismal future for malaria, which thrives in a warm environment. There are ongoing efforts by the World Health Organization, or WHO, to prevent and reduce malaria illness.
Unfortunately, as a harbinger of the problems with antimicrobial resistance, mosquitoes have developed resistance to treatment drugs from widespread misuse. The mosquitoes also seem resilient to widespread pesticide applications, even Dichlorodiphenyltrichloroethane, more commonly known as DDT.
This is a preventable and curable illness that is still causing high death rates. Even with the advances in medicine, new diagnostic tools, and treatment, malaria continues to be a deadly disease.
Common Questions about the Germ Theory and Malaria
The discovery of the microscope five centuries after the plague epidemic led scientists and doctors to identify the microscopic organisms as the culprits that spread infectious diseases.
The plague was hard to eliminate due to the lack of knowledge of germ theory and the lack of tools for the diagnosis of illnesses.
Malaria is a parasite transmitted by mosquito bites that affects the functioning of the red blood cells that control the delivery of oxygen to the body.