Ever since our conception, humans have fallen victim to infectious disease - microscopic, airbourne pathogens and parasites that infiltrate our bodies and turn them against us. Shown above, and described below, are 10 of the deadliest pathogens humankind has encountered throughout history. Some, like poliovirus, show how far we’ve come - while others, such as HIV, remind us how far we have still to go in the battle against nature’s smallest assassins.
The Bubonic Plague: Also called the Black Death due to the formation of necrotic tissue on living victims, the bubonic plague - most commonly caused by a small bacterium, Yersinia pestis - is estimated to have killed around 75 million people, including half the total population of Europe. Although controlled, the bubonic plague is still endemic today.
Poliomyelitis: One of the most dreaded childhood diseases of the 20th century, the causitive agent of polio, poliovirus, has caused 10,000 deaths since 1916, and permanent paralysis to thousands. Its presence in the population is substantially reduced in the modern day due to an effective polio vaccine and vaccination programme.
Smallpox: Marked in history as the pathogen of choice for the first-ever documented case of biological warfare, in which smallpox-infected blankets were thrown into enemy camps, smallpox and its two viral agents - variola major (pictured above) and variola minor - decimated the Native American population in the United States from 12 million to 235,000. It is also credited with destroying the Aztec civilisation when brought to South America by the conquistadors. WHO declared the official eradication of smallpox in 1979, although samples are still stored in laboratories for research.
Cholera: Caused by the bacterium Vibrio cholerae, cholera is perhaps best known for being one of the most rapidly fatal illnesses known - a healthy person may become hypotensive within an hour of symptoms onset, and will die within 2-3 if no treatment is provided. Cholera has killed approximately 12,000 people since 1991.
Spanish Influenza: An especially virulent strain of Influenza A virus, subtype H1N1, killed 50 to 100 million people in the years 1918 and 1919 alone. Many of its victims were healthy young adults, in stark contrast to the flu of today, which usually preys on the old and infirm. The extraordinary death toll is believed to have resulted from the extreme virulence of the virus and the severity of symptoms, believed to have been caused by cytokine storms.
Tuberculosis: Caused by various strains of mycobacteria, most commonly Mycobacterium tuberculosis, tuberculosis is a usually lethal and sadly common infectious disease that affects up to 80% of the population in some African and Asian countries.
Influenza: Commonly known as the flu, influenza is caused by a massive family of RNA-based viruses of the family orthomyxoviridae. It causes about 36,000 deaths per year.
Malaria: Malaria is a vector-bourne infectious disease caused by protozoan parasites of the genus Plasmodium, typically Plasmodium falciparum and Plasmodium vivax. It causes approximately 2.7 million deaths per year, a large percentage of them young children in sub-Saharan Africa. No vaccine has yet been created for malaria; drugs must be taken continuously to reduce the risk of infection.
AIDS: Acquired Immunodeficiency Syndrome (AIDS) is caused by HIV, the human immunodeficiency virus. Death results from specific damage to the immune system, leaving people susceptible to opportunistic infection in the late stages. Although treatments exist to decelerate the virus’ progression, there is no known cure, and 21 million have died of AIDS since 1981. HIV is usually passed by blood-to-blood transmission.
Ebola: Ebola is a potentially lethal hemorrhagic fever that has caused approximately 1,600 human deaths. It is a zoonotic disease caused by the ebola virus whose primary animal vector is thought to be the fruit bat. Mortality rates are generally very high, in the region of 80% – 90%, with the cause of death usually due to hypovolemic shock or organ failure.
Images: Top left: Yersinia pestis. Top right: poliovirus. Second line, left: Variola major. Second line, center: Vibrio cholerae. Second line, right: Influenza A, subtype H1N1. Third line, left: Mycobacterium tuberuclosis. Third line, right: Influeza A. Bottom left: Plasmodium falciparum in red blood cells. Bottom center: HIV. Bottom right: Ebola virus.