The 20 Deadliest Infectious Diseases in Human History

Smallpox was a highly contagious and deadly disease that ravaged humanity for thousands of years. Causing a high fever and a characteristic rash, the virus claimed the lives of approximately 300 million people in the 20th century alone. In 1980, following a global vaccination campaign, smallpox became the first human infectious disease to be declared eradicated by the World Health Organization.

The bubonic plague was a pandemic that ravaged Europe, Asia, and North Africa in the 14th century, claiming the lives of an estimated 75 to 200 million people. The plague was caused by the bacterium Yersinia pestis, which spread primarily through fleas on rodents. The pandemic had a profound impact on society, reshaping economies and demographic structures.

Malaria is an infectious disease transmitted by the bites of infected mosquitoes. It has claimed tens to hundreds of millions of lives throughout history and continues to cause serious illness today, particularly in tropical and subtropical regions. Before the advent of modern medicine, malaria was often fatal, especially for young children.

The Spanish flu infected about one-third of the world’s population between 1918 and 1920. It is estimated that this flu pandemic claimed between 40 and 50 million lives worldwide, although some estimates are much higher. Unlike other flu epidemics, it struck healthy young adults with alarming frequency.

COVID-19, or coronavirus disease 2019, emerged in late 2019 and quickly became a pandemic. As of mid-2020, the number of confirmed deaths worldwide stood at several million, with estimates of excess mortality being much higher. Caused by the SARS-CoV-2 virus, the disease is transmitted primarily through respiratory droplets.

The human immunodeficiency virus (HIV) was identified in the early 1980s, but the disease it causes, acquired immunodeficiency syndrome (AIDS), has since claimed more than 40 million lives. The virus weakens the immune system, making the body vulnerable to other infections. Although there is no cure, antiretroviral therapy allows many people to lead long and healthy lives, and the mortality rate has dropped significantly thanks to increased access to treatment.

Tuberculosis, also known as TB, has been one of the leading causes of death for centuries. It is estimated to have claimed the lives of more than one billion people throughout human history. The disease primarily affects the lungs and spreads through the air.

Cholera has caused numerous pandemics since the 19th century, claiming the lives of millions of people. The disease is transmitted through contaminated water and food and causes severe dehydration. If left untreated, it can lead to death within a few hours.

Typhus has been responsible for the deaths of several million people, particularly during wars and periods of social unrest. It is a disease transmitted by lice that spreads under conditions of overcrowding and poor hygiene. Historically, typhus epidemics have weakened armies and civilian populations.

Measles is one of the most contagious diseases known to science. Before widespread vaccination, it claimed the lives of millions of people worldwide, particularly children. Its symptoms include fever and skin rashes and can lead to serious complications such as pneumonia. Global vaccination efforts have saved countless lives.

Yellow fever is a viral disease that has caused numerous major epidemics in Africa and the Americas over the course of several centuries. Transmitted by mosquitoes, it can lead to severe liver disease. Some of the largest epidemics in history occurred in the 18th and 19th centuries, claiming the lives of hundreds of thousands of people in major cities.

The flu, or influenza, has been a major cause of illness and death, leading to numerous deadly pandemics throughout history. In addition to these major epidemics, seasonal flu strains also cause regular, annual outbreaks of illness. Influenza viruses are easily transmitted through the air and undergo frequent mutations.

Hepatitis B is a viral infection that attacks the liver and can develop into a chronic, lifelong disease. Over time, it has claimed the lives of millions of people who have died from liver failure and cancer. The virus is transmitted primarily through contact with infected blood or bodily fluids.

Hepatitis C is another major cause of chronic liver disease worldwide. This disease has been responsible for millions of deaths over the past few decades and can progress silently for years. The virus is transmitted primarily through exposure to infected blood.

Whooping cough, or pertussis, was responsible for widespread infant mortality before vaccines were developed. This highly contagious respiratory disease causes violent coughing fits and can lead to serious complications. Millions of children died of whooping cough in the 19th and early 20th centuries.

Polio caused widespread paralysis and numerous deaths during major epidemics in the 20th century. Although the disease is not always fatal, severe cases that result in paralysis can lead to life-threatening complications. Vaccination campaigns have nearly eradicated the disease.

The Ebola virus has caused several deadly outbreaks since its discovery in 1976. While the total number of victims is far from matching that of certain historical diseases, the outbreaks have nonetheless resulted in very high mortality rates. The virus is transmitted through direct contact with the bodily fluids of infected individuals.

Severe Acute Respiratory Syndrome (SARS) was an infectious disease that first emerged in 2002 and spread to other countries within a few months. The number of deaths was much lower than that of COVID-19, but the case fatality rate was high. Prompt public health measures helped contain the outbreak.

Meningococcal infections have caused numerous deadly epidemics throughout history. Meningococcal disease can lead to both meningitis and bloodstream infections. If left untreated, the disease can progress rapidly.

Leprosy has affected humanity for thousands of years, causing widespread disabilities and social stigma. It is not always fatal, but historically, many patients with leprosy have died from complications and secondary infections. The disease spreads slowly and is now fully treatable with medication.