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Infectious diseases, alongside natural disasters, have been among the most significant unintended causes of human mortality and suffering across the globe. These diseases have often left a lasting impact, altering the trajectory of human history.
In some instances, such as with the bubonic plague, population numbers were drastically reduced for centuries. In other cases, like polio, the illness of a famous individual brought more attention to the disease, highlighting the urgent need for a cure.
10. Bubonic Plague
During the 14th century, the bubonic plague (commonly known as the “Black Death”) swept across Europe from east to west. The disease was caused by the bacterium Yersinia pestis, which was transmitted by Oriental rat fleas. These fleas, carried by rats, spread the plague as the rodents traveled westward along the Silk Road and across Mediterranean ships.
The bubonic plague highlighted the role of human progress in trade and commerce in facilitating the rapid spread of deadly pathogens. Its name is derived from the Latin word bubo, which refers to a swollen, infected lymph node or abscess.
The symptoms were horrifying. They began with fever and perspiration, eventually developing into blackish-blue boils on the groin. If untreated, these boils would enlarge, causing death from toxic accumulation. Even attempts to drain the boils were often fatal, as it could lead to the bacteria becoming airborne.
The plague's mortality rate exceeded 70 percent, claiming the lives of up to 200 million people in Europe, halving the continent’s population. It is believed that the bubonic plague contributed to the downfall of the feudal economy and caused significant harm to the church.
Many priests became infected after performing last rites and conducting funeral services. Others chose to abandon their parishes, fearing they might catch the plague. The bubonic plague continues to be regarded as one of history's most horrific diseases, even though the advent of antibiotics has reduced the frequency of the Black Death in modern times.
9. Smallpox
When Europeans first set foot in the New World in the late 15th and early 16th centuries, they brought with them not only advanced military strategies but also the deadly smallpox virus. This disease played a key role in the devastating loss of life among Native Americans.
In the Old World, Europeans had long lived in close quarters with domesticated animals and shared food and water sources, fostering the spread of various diseases. Those who survived these outbreaks built up an impressive immunity to otherwise fatal pathogens. Many of these immune individuals were among the first settlers of America, introducing smallpox to the continent as early as 1520.
Along with other Old World diseases such as the flu and measles, smallpox devastated nearly 90 percent of the Native American population, far surpassing the destruction caused by late medieval warfare. Smallpox was not only deadly but also left its survivors with unsightly scars and sores across their bodies.
Several centuries later, smallpox became one of only two diseases (the other being rinderpest) to be completely eradicated from the human population, thanks to vaccination efforts. Today, the disease exists only in highly secured laboratory environments.
8. Spanish Influenza
The 1918 flu pandemic was caused by one of the deadliest pathogens of the 20th century, infecting around 500 million people worldwide. What began as outbreaks in the United States and Europe quickly spread across the globe.
While this lethal strain of the flu wreaked havoc across population centers, it came to be known as “Spanish influenza” because Spain was particularly hit hard by the virus. Even King Alfonso XIII of Spain became infected.
The Spanish influenza had a profound effect on the battlefields of World War I, as it spread rapidly among many young and otherwise healthy soldiers. Historical records suggest that more Americans died from the 1918 flu than from combat on the front lines.
Around 40 percent of the US Navy contracted the flu, with nearly as many soldiers in the US Army also afflicted. The flu’s consequences were felt across both the economy and the military, with many believing it had a significant impact on the course of World War I by infecting militias and crippling medical infrastructure.
The development of flu vaccines began in the 1940s.
7. Polio
In today's world, polio is extremely uncommon, with only a few cases appearing in developed countries after Jonas Salk's groundbreaking creation of the polio vaccine. Before this medical breakthrough, polio was easily spread through an infected person’s stool or airborne droplets when sneezing.
Polio often shows no symptoms. However, when they do occur, they can be crippling. The disease is infamous for causing paralysis, leaving its victims to spend the rest of their lives relying on iron lungs. Once paralysis sets in, it cannot be undone. Although portable generators may help some, others remain dependent on the classic iron lung machines, which gained prominence in the 1940s.
The most famous person affected by polio was Franklin D. Roosevelt, the former President of the United States. His efforts to lessen the social stigma surrounding the disease had a profound impact on the presidency during and after World War II. These actions shifted the way both individuals and the nation viewed paralysis-related illness and disability.
6. Syphilis
Syphilis, a sexually transmitted infection, progresses through four stages. It begins with a painless chancre at the site of infection. In the second stage, syphilis manifests as a widespread rash and swollen lymph nodes. The infection then enters a dormant phase before resurfacing as tertiary syphilis, which can result in neuromuscular degeneration, blindness, and dementia.
The origins of syphilis in European populations remain unclear, but the most widely accepted theory is that it was introduced through New World colonization. Many notable historical figures have been affected by syphilis, including various members of the medieval papacy. In 1508, Pope Julius II was unable to have his foot kissed due to syphilitic sores covering it.
Throughout history, syphilis has led to the use of ineffective and harmful treatments, such as mercury, which often worsened the condition. Despite its persistence, syphilis can now typically be cured with penicillin.
5. HIV/AIDS
Few diseases have carried as much stigma as the human immunodeficiency virus (HIV), which later progresses to acquired immunodeficiency syndrome (AIDS). It is believed that the virus crossed from primates to humans in Africa in the early 20th century. However, it wasn't until the early 1980s, when several gay men in New York and California developed unusual cases of pneumonia and cancer, that the disease entered public awareness.
In the early years of the epidemic, HIV was associated with gay men, leading to the term 'gay-related immune deficiency' (GRID). Paranoia spread across Europe and the United States as the public was uncertain about how the disease was transmitted. This connection between HIV and the gay community prompted the formation of activist groups like ACT UP, which played a key role in advancing LGBT rights and contributing to the eventual recognition of sexual minorities' rights decades later.
4. Tuberculosis
Tuberculosis (TB) is a severe lung infection that comes in two forms: latent TB and active TB. Latent TB is not contagious, and most people’s immune systems can control it. In fact, one-third of the world’s population carries latent TB.
When the immune system is weakened, active TB can take hold. Symptoms include persistent coughing, intense chest pain, night sweats, and loss of appetite. The rise of HIV/AIDS has contributed to an increase in TB cases, as individuals with compromised immune systems struggle to defend against the otherwise dormant bacteria.
Tuberculosis has had a significant impact on science, especially in the 19th century, when it was often transmitted through milk. This led to the creation of batch pasteurization, a low-temperature pasteurization method that originated from efforts to eliminate tuberculosis from dairy products.
3. Cholera
At its most severe, cholera can progress from being asymptomatic to fatal in under three hours. Cholera is a diarrheal illness caused by a bacterium, typically spread via contaminated water or food in areas lacking proper sanitation. While the disease's origins are traced to the Ganges Delta in India, it has since spread globally. Cholera has caused pandemics in South Asia (1961), Africa (1971), and the Americas (1991).
Every year, approximately four million people contract cholera, with over 100,000 fatalities. On July 28, 2010, the United Nations officially acknowledged clean drinking water as a fundamental human right, directly linking this decision to the spread of waterborne pathogens.
2. Ebola
Few illnesses have stirred as much panic as Ebola, a virus that emerged in Africa in the late 1970s. Ebola, short for Ebola hemorrhagic fever (EHF), causes severe bleeding in humans and primates. The virus's symptoms can take several days to weeks to manifest, including sore throat, muscle pain, vomiting, diarrhea, and ultimately both internal and external bleeding.
Depending on the strain, Ebola carries a high mortality rate, killing nearly half of those it infects. In some cases, this rate can soar up to 90 percent. The most devastating Ebola outbreak began in West Africa in March 2014, claiming five times as many lives as all prior outbreaks combined.
The virus spread to the United States and Europe, affecting countries like the United Kingdom, France, Germany, and Spain. Full containment wasn't achieved until 2016. The outbreak posed a major challenge to the World Health Organization, testing its ability to handle a modern pandemic.
1. Malaria
Malaria is a disease transmitted by mosquitoes, caused by parasites that induce flu-like symptoms in those affected. It remains one of the deadliest diseases in the world, infecting over 200 million people in 2016 and causing nearly 500,000 deaths.
Malaria is often remembered for its possible role in the death of Alexander the Great. But did you know that malaria—and the resistance to it—played a crucial role in the tragic transatlantic slave trade? There is no evidence that malaria existed in precolonial America, and its introduction decimated native populations.
This led early European settlers to turn to African people, who were often resistant to malaria if they had survived the disease in childhood, to work on plantations in the American South. As malaria ravaged the region, decimating the Native American population, Africans were tragically forced into the transatlantic slave trade.
On the medical front, malaria provided scientists with crucial insights into transmission vectors, including how animals that carry diseases can spread them to human populations, and what measures can be taken to prevent such infections.
