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  • Writer's pictureP.K. Peterson

The Columbian Exchange: Microbes That Conquered the Americas

“The Old World invaders came in with a raft of infectious diseases. Not that the New World didn't have any at all, but it did not have the numbers that were brought in from the Old World.”

Alfred W. Crosby, American historian, author of The Columbian Exchange 

We are very lucky to be living in an age in which we are still making discoveries. It is like the discovery of America-you only discover it once."

Richard P. Feynman, American theoretical physicist, Nobel laureate, author


Historian Alfred W. Crosby coined the term “Columbian Exchange” in his 1972 book The Columbian Exchange: Biological and Cultural Consequences of 1492. Named after the Italian explorer Christopher Columbus, the term refers to the widespread transfer of plants, animals, precious metals, culture, human populations, technology, ideas, and diseasesbetween the New World (the Americas) in the Western Hemisphere, and the Old World (Afro-Eurasia) in the Eastern Hemisphere in the late 15th and 16th centuries.

While the late 15th and 16th centuries may have been an exciting time for the explorers, it was doomsday for the indigenous people in the Americas who had never been exposed to a large number of pathogens (viruses, bacteria and parasites) that the explorers carried with them from Europe. In this week’s Germ Gems post, I identify the microbes that helped the explorers conquer the Americas.

Fellow travelers: pathogens that accompanied the explorers.  The European explorers carried to the Americas viruses, bacteria and parasites that were unknown to the Native peoples against which they lacked immunity. Analyses of historical records indicate that at least three viral infections—smallpox, measles, and influenza—were the major culprits. Of these, smallpox was “Public Enemy Number 1.” (See September 23, 2020 Germ Gems post, “Smallpox Is Gone, But the Lessons Learned Live On”).

The explorers also transmitted the following three highly pathogenic bacterial species to Natives in the New World:Rickettsia prowazekii, the cause of typhus; Corynebacterium diphtheriae, the cause of diphtheria; and Bordetella pertussis, the etiology of whooping cough. All of these diseases spread rapidly as the indigenous populations had no immunity to these never-encountered-before pathogens. Consequently, the mortality rate was extremely high. It is estimated that these pathogens decimated between 80-95% of the Native American population within the first 100-150 years following Columbus’s 1492 voyage.

The only parasite that figures into the Columbian Exchange is Plasmodium falciparum— the cause of malignant tertian malaria. Malaria was introduced into the Americas in the 16th-century in the blood of parasitized slaves and settlers. The vector, the Anopholes mosquito, was already here ready to spread the parasite widely throughout the Americas causing a huge number of deaths of both Natives and Europeans. (Many researchers believe that malaria originated and coevolved with humans in Africa, but that its spread across the world can be blamed on colonization.)

The payback? Treponema pallidum is the bacterium that causes syphilis—a disease that continues to wreak havoc in both the Old World and the New World.  Whether syphilis originated in the New or the Old World is a matter of controversy.


According to some scholars, navigators in Christopher Columbus’s fleet brought this affliction from the New World back to the Old World on their return in 1493 as the disease was unknown in Europe until the late 15th century. Some have considered syphilis a justifiable payback, given that the explorers carried such an astounding number of highly lethal pathogens with them to the Americas.


At its beginning in Europe, syphilis was a severe, often deadly disease. Over time the disease evolved, becoming more benign and less fatal.


Lessons learned from the Columbian Exchange. The major take-away lesson from the Columbian Exchange is the extraordinary importance of human discovery. This is especially so in the field of science.


The indigenous people of the Americas had no immunity to many of the diseases that plagued the Old World in the 15thcentury. Scientists then discovered the “new” world of microbes (Leeuwenhoek in 1676) and “the “germ theory of disease” (Louis Pasteur and Robert Koch in the 19th century). These discoveries and further scientific exploration led ultimately to the development of vaccines and antibiotics that can contain, if not control, all of the diseases that were brought to the New World in the Columbian Exchange.

But new pathogens are emerging and being discovered all the time. (At least 40 new infectious diseases have been discovered since the 1970s.) Travel continues to be a critical factor in the dissemination of these emerging pathogens.

The vehicles transporting humans carrying lethal pathogens has changed from ships in the 15th century to our jet planes today thereby enabling a broader and quicker dissemination of pathogens. And, the lack of protective immunity in populations threatened by new pathogens continues to play a central role in the outcomes of infectious diseases. Perhaps there is no better example of this than SARS-CoV-2, the virus that emerged in 2019 and spread rapidly across the globe causing the COVID-19 pandemic.

We humans had never encountered this novel coronavirus before and thus we had no immunity to it. The World Health Organization (WHO) estimates that between January 2020 and the end of December 2021, 14.9 million people died globally from COVID-19.

Fortunately, scientific discoveries came to the rescue. In remarkably short order, researchers developed new mRNA vaccines that were safe and effective in reducing the deaths from COVID-19. The WHO reports that since their introduction, vaccines reduced COVID-19 deaths in the WHO European region by 57%, saving more than 1.4 million lives.

While we may have the COVID-19 pandemic under control, we must remain vigilant; our battle against microbes is on-going. In the 19th century, Louis Pasteur, the father of modern microbiology, cautioned,It is the microbes who will have the last word.” I certainly hope not. Instead, I hope that the discoveries of brilliant scientific explorers will enable humans to have a very long-lasting, if not the absolutely final word about this existential matter.

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Main Page images courtesy of Shuxian Hu, MD. Dr. Hu is a scientist in the Neuroimmunology Research Laboratory at the University of Minnesota.

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