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World's deadliest diseases: 7 diseases that impacted human history

Dr. Ajay KohliFeb 15, 2023

Throughout history, many notable epidemics and pandemics have ravaged civilizations, threatening human existence. But, thanks to the advances made towards virology, surveillance, drug discovery, and vaccine development, humans persevered and survived. Here’s an overview of some diseases that altered the course of human history and healthcare forever.

World's Deadliest Diseases



1. Bubonic Plague

Bubonic Plague is a potentially fatal infectious disease caused by the bacterium, Yersinia pestis. Throughout centuries, the disease has erupted several times in different eras, claiming between ten and millions of lives worldwide. However, the devastation reported in the 14th century, also called "The Black Death, " was one of mankind's worst pandemics that killed more than 25 million people, including a third of Europe's population. This ultimately led to the use of quarantine as the primary public health measure. 

Even today, plague outbreaks occur throughout the world, mainly spreading through the bite of an infected flea that lives on small animals such as gerbils, rats, marmots, and squirrels. Symptoms appear within three to five days after the flea bite and include:

  • sudden fever

  • headache

  • chills 

  • weakness

  • swollen, painful lymph nodes or buboes that leak pus (especially near the site of infection)

Lung infections, vomiting of blood, and scattered black spots can also be experienced. Plague kills 30 to 90% of those infected without treatment within 10 days of bite exposure. But with intensive antibiotic treatment, the risk drops to 10%. The best way to prevent the spreading of Bubonic Plague is to avoid handling live or dead animals and use insect repellents containing DEET, among other precautions.

Bubonic Plague

2. Spanish flu or Influenza

Influenza or flu is a contagious respiratory disease caused by variants of Influenza viruses. For more than a century, there have been six major Influenza epidemics. But the 1918 to 1920 flu pandemic, popularly known as "the Great Influenza epidemic" or "the Spanish flu," was the most severe, caused by the H1N1 virus with genes from avian origin. The disease killed about fifty million people, mostly young adults, and infected another 500 million amid World War I. In 2009, a relatively mild flu outbreak killed approximately 3,00,000 people.

Humans are host to several types of Influenza viruses, but pandemics occur only when a new strain is transmitted to humans by other animals, such as pigs, ducks, or chickens. Symptoms of the flu range from mild to severe and usually include:

  • fever

  • runny nose

  • sore throat

  • body ache

  • headache

  • coughing

  • watery red eye

  • tiredness

Currently, Influenza isn't preventable by vaccines. So, most controlling efforts are non-pharmaceutical, limited to isolation, quarantine, good personal hygiene, disinfectants, and limiting public gatherings.

Spanish flu or Influenza

3. Smallpox

Smallpox, a contagious viral infection caused by the variola virus, has been one of the most widespread causes of human death for centuries. Variola major and variola minor were the two types of Smallpox virus, with variola major being the severe form. The disease was more common in children, but the risk seemed higher for those who came in contact with an infected person or contaminated objects. People who had Smallpox usually had the following symptoms.

  • fever

  • vomiting

  • skin rashes

  • fluid-filled bumps

While the first mention of Smallpox dates back to the reign of the great Egyptian Pharaoh Ramses V (1156 BC), the outbreak in 18th-century Europe was the deadliest, killing 30% of those infected, mostly babies. Those who survived had extensive body scars, and about a third were left blind. In the 20th century alone, the disease killed around 300 million people. Smallpox was last naturally endemic in 1977. Since then, the success of vaccination has eradicated disease globally.


4. Cholera

Cholera is an acute intestinal infection caused by the bacterium Vibrio Cholera. The illness is marked byprofuse cramping, vomiting, and watery diarrhoea, leading to rapid dehydration. If left untreated, the symptoms turn so severe that patients usually die within hours. Cholera spreads through contaminated food and water, often causing small outbreaks. However, these outbreaks quickly become a crisis if sanitation systems are disrupted.

In the twenty-first century, the illness continues to affect approximately 2.9 million people per year, resulting in 95,000 deaths worldwide, mostly in low- and middle-income countries due to poverty. The African continent, in particular, has been hit hard, with 40 million people living in Cholera-endemic areas at risk of frequent outbreaks. At the same time, more developed countries such as North America and Europe have had virtually no Cholera for a century due to improved sanitation infrastructure and advances in personal hygiene.



HIV, or human immunodeficiency virus, which causes acquired immunodeficiency syndrome, or AIDS, remains one of the fatal diseases of the 21st century, killing tens of millions since 1981. There are currently 38.4 million people living with HIV/AIDS worldwide, of which over 7 million come from Sub-Saharan Africa.

For decades the illness had no cure. But with awareness and the advent of antiretroviral treatment, the disease has become more manageable, with global HIV death rates dropping from 2.2 million to 1.6 million between 2005 and 2012.

HIV can be transmitted through unprotected sexual contact, intravenous drug use, infected blood transfusion, shared use of injectors, from mother-to-child transmission during pregnancy, childbirth, and breastfeeding. The virus destroys a type of white blood cell essential to a functioning immune system. But as the virus gradually weakens the immunity, it causes symptoms, making it harder for your body to resist infections and other diseases.

HIV Aids

6. Ebola

Ebola is a rare and fatal disease caused by the Ebola filovirus. There are five types of Ebola virus, four of which are known to cause human disease. The Bat is suspected to be the natural reservoir of the virus. Humans contract the disease through direct contact with vomit, infected body fluids, or contaminated objects such as needles and syringes. Symptoms usually begin between two and 21 days after infection and include:

  • fever

  • sore throat

  • severe headaches

  • diarrhoea

  • vomiting

  • muscle pain and weakness

  • decreased liver and kidney function

  • bleeding and bruising (both internally and externally)

Several small outbreaks of Ebola have occurred in Africa since its discovery in 1976, but the incident between 2013 and 2016 was the deadliest. The episode began in Guinea and progressed to Sierra Leone and Liberia. Roughly 28,600 people got infected, and 11,325 died before the virus was contained and declared in 2016.


7. Coronavirus

Initially reported in Wuhan in late 2019, the coronavirus illness is caused by the novel SARS-CoV-2 virus. The disease quickly spread like wildfire, resulting in a pandemic. As of mid-October 2022, approximately 6.5 million people had died from the COVID -19 pandemic.

People contract COVID-19 when they inhale virus-containing droplets/aerosols and tiny airborne particles expelled by infected people while coughing, sneezing, or speaking. Symptoms typically appear two to 14 days after virus exposure, with most people tending to have mild to moderate symptoms such as fever, loss of smell, and tiredness. However, severe symptoms may be seen in those with other health problems such as diabetes, cardiovascular diseases, etc. There is no standard cure for COVID-19, but there are many ways to manage it, including oxygen support, antivirals, and, most importantly, vaccines.



Disclaimer: The content on this page is generic and shared only for informational and explanatory purposes. Please consult a doctor before making any health-related decisions.


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