At one time or another, humans have turned to just about every viable option on the planet for new means of destroying one another. We've leveled forests, plundered the elements and diverted religion, philosophy, science and art to fuel humanity's desire for bloodshed. Along the way, we've even weaponized some of nature's most formidable viral, bacterial and fungal foes.

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The world is home to a number of deadly viruses, bacteria and fungi. Biological warfare involves the human use of these agents to destroy one another. See more pictures of guns.
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The use of biological weapons, or bioweapons, dates back to the ancient world. As early as 1,500 B.C. the Hittites of Asia Minor recognized the power of contagions and sent plague victims into enemy lands. Armies, too, have long understood the power of bioweapons, catapulting diseased corpses into besieged fortresses and poisoning enemy wells. Some historians even argue that the 10 biblical plagues Moses called down against the Egyptians may have been more of a concentrated campaign of biological warfare rather than the acts of a vengeful god [source: NPR].
Since those early days, advances in medical science have led to a vastly improved understanding of harmful pathogens and the way our immune systems deal with them. But while these advancements have led to vaccinations and cures, they have also led to the further weaponization of some of the most destructive biological agents on the planet.
The first half of the 20th century saw the use of the biological weapon anthrax by both the Germans and Japanese, as well as the subsequent development of biological weapons programs in nations such as the United States, the United Kingdom and Russia. Today, biological weapons are outlawed under 1972's Biological Weapons Convention and the Geneva Protocol. But while a number of nations have long destroyed their stockpiles of bioweapons and ceased research into their proliferation, the threat remains.
In this article, we'll examine some of the leading bioweapon threats, as well as what the future of biological warfare may have in store for us all.

Bioweapon 10: Smallpox

An Ottawa tribe member confronts the British colonel who spread smallpox with infected blankets.
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The term "biological weapon" typically summons mental images of sterile government labs, hazmat suits and test tubes full of brightly colored liquid apocalypse. Historically, however, biological weapons have often taken much more mundane forms: a wandering exile, paper bags full of plague-infested fleas or even, during the1763 French and Indian War, a simple blanket.
At the orders of Cmdr. Sir Jeffrey Amherst, British forces infamously distributed smallpox-infected blankets to Native American tribes in Ottawa. The native inhabitants of the Americas were particularly susceptible to the illness since, unlike their European invaders, they hadn't encountered smallpox before and lacked any degree of immunity to it. The disease cut through the tribes like wildfire [source: Yount].
Smallpox is caused by the variola virus. The most common form of the disease has a 30 percent mortality rate [source: CDC]. Signs of smallpox include high fevers, body aches, and a rash that develops from fluid-filled bumps and scabs to permanent, pitted scars. The disease predominantly spreads through direct contact with an infected person's skin or bodily fluids, but also can be spread though the air in close, confined environments.
In 1967, the World Health Organization (WHO) spearheaded an effort to eradicate smallpox through mass vaccinations. As a result, 1977 marked the last naturally occurring case of smallpox. The disease was effectively eliminated from the natural world, but laboratory copies of smallpox still exist. Both Russia and the United States possess WHO-approved stores, but as smallpox played a role in several nations' bioweapons programs, it's unknown how many secret stockpiles still exist.
The CDC classifies smallpox as a Category A biological weapon due to its high mortality rate and the fact that it can be transmitted through the air. While a smallpox vaccine exists, typically only medical and military personnel undergo vaccination -- meaning the rest of the population is very much at risk if smallpox were unleashed as a weapon. How might the virus be released? Probably in aerosol form or even in the old-fashioned way: by sending an infected individual directly into the target area.
The method for unleashing a biological weapon doesn't have to be flashy, however. Consider how much press our next bioweapon received, all with a few postage stamps.

Category A Biological Weapons
The U.S. Centers for Disease Control and Prevention classifies a Category A biological weapon as any easily spread microorganism or toxin that has a high potential for casualties, might cause public panic and requires special public-health preparedness. Only six biological agents currently meet these criteria, all of which we thought deserved to be named scary bioweapons [source: CDC].

Bioweapon 9: Anthrax

Hazardous materials experts enter the Hart Building of the U.S. Senate after an anthrax-laced letter was found in Sen. Tom Daschle's office.
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During the fall of 2001, letters containing a curious white powder began turning up at U.S. Senate offices and media outlets. When word spread that the envelopes contained the spores of the deadly bacteria Bacillus anthracis, panic ensued. The anthrax letter attacks infected 22 people and killed five. Seven years later, the FBI finally narrowed down its investigation to government anthrax scientist Bruce Ivans, who committed suicide before the case could be closed.
Thanks to its high mortality rate and environmental stability, the anthrax bacteria is also classified as a Category A biological weapon. The bacteria live in the soil, where grazing animals typically come into contact with spores while rooting around for food. People, however, may become infected with anthrax by touching the spores, inhaling them or ingesting them.
Most cases of anthrax are cutaneous, transmitted through skin contact with the spores. The most deadly form is inhalation anthrax, when the spores travel to the lungs and then the immune cells carry them to the lymph nodes. Here, the spores multiply and release toxins that result in such symptoms as fever, respiratory problems, fatigue, muscle aches, enlarged lymph nodes, nausea, vomiting, diarrhea and black ulcers. Inhalation anthrax carries the highest mortality rate of the three (100 percent, 75 percent with medical treatment), and unfortunately, that was the form contracted by all five casualties from the 2001 anthrax letters [source: NPR].
The disease isn't easy to catch under normal situations, and it can't be transmitted from person to person. Still, health workers, veterinarians and military personnel normally undergo vaccinations. The rest of us, however, remain at risk if someone were bent on another anthrax attack.
Along with the lack of widespread vaccination -- a common theme among our scary bioweapon nominees -- longevity is another point in anthrax's favor. Many harmful biological agents can only survive a short while under certain conditions. But hardy B. anthracis can sit on the shelf for 40 years or more and still pose a lethal threat.
These attributes helped to establish anthrax as a favorite among bioweapons programs throughout the world. Japanese scientists conducted human experiments with aerosolized anthrax in the late 1930s in their infamous Unit 731 biological warfare facility in occupied Manchuria. British forces experimented with anthrax bombs in1942, managing to so thoroughly contaminate test site Gruinard Island that, 44 years later, 280 tons of formaldehyde were required to decontaminate it. In 1979, the Soviet Union accidently released airborne anthrax, killing 66 people in the process.
Today, B. anthracis remains one of the most well-known and feared bioweapons. Numerous biological warfare programs have worked to produce anthrax over the years and while a vaccine exists, mass vaccination would only become viable if mass exposure occurred.
We don't even have a vaccine for some bioweapons. The only way to avoid our next entry is to avoid exposure.

Bioweapon 8: Ebola Hemorrhagic Fever

A nurse dons protective clothing in the Ebola isolation ward of a Ugandan hospital.
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Another well-documented killer exists in the form of the Ebola virus, one of more than a dozen different viral hemorrhagic fevers, nasty illnesses sometimes marked by copious bleeding. Ebola began to make headlines in the late 1970s as it spread through Zaire and Sudan, killing hundreds. In the decades that followed, the virus maintained its lethal reputation in outbreaks across Africa and proved a volatile organism even in controlled settings. Since its initial discovery, no fewer than seven outbreaks have occurred at hospitals and laboratories in Africa, Europe and the United States.
Named for the region of the Congo in which it was first discovered, scientists suspect the Ebola virus normally resides within a native, African animal host, but the exact origin and natural habitat of the disease remain a mystery. As such, we have only encountered the virus after it has successfully infected humans or nonhuman primates.
Once present in a host, the virus infects others through direct contact with blood or other bodily secretions. In Africa, the virus has proved itself particularly adept at spreading through hospitals and clinics. An infected individual can expect to start experiencing symptoms in between 2 and 21 days. Typical symptoms may include headache, muscle ache, sore throat and weakness, followed by diarrhea and vomiting. Some patients also suffer internal and external bleeding. Between 60 and 90 percent of infections end in death after 7 to 16 days [source: Chamberlain].
Doctors don't know why some patients are better able to recover than others. Nor do they how to treat it. And, as noted earlier, there's no Ebola vaccine. In fact, we only process a vaccine for one form of hemorrhagic fever: yellow fever.
While many medical professionals labored to better treat and prevent outbreaks of Ebola, a team of Soviet scientists set out to turn the virus into a weapon. They initially encountered difficulties cultivating Ebola in the laboratory, enjoying more success with the development of Marburg hemorrhagic fever. By the early 1990s, however, they had solved the problem [source: Alibek]. While the virus normally spreads through physical contact with bodily secretions, researchers have observed it spread through the air under laboratory conditions. The possibility of a weaponized, aerosol form of the virus only further cements Ebola and related viral hemorrhagic fevers as permanent placeholders on the list of Category A agents.
The word "Ebola" is already synonymous with terror and death, despite having only become news in the last few decades. Our next entry, however, has been plaguing humans for centuries.

Bioweapon 7: Plague

To understand the psychological impact of plague, you don't have to look much farther than this detail from Pieter the Elder Brueghel's 1562 work "The Triumph of Death."
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The Black Death decimated half the population of Europe in the 14th century -- a horror that continues to resonate through the world even today. Dubbed "the great dying," the mere prospect of a return to such times is enough to put a population on edge. Today, some researchers speculate that the world's first pandemic may have actually been a hemorrhagic fever, but the term "plague" continues to cling to another long-standing suspect and current Category A biological weapon: the Yersinia pestis bacterium [source: MacKenzie].
Plague exists in two main strains: bubonic and pneumonic. Bubonic plague typically spreads by bites from infected fleas, but also can be transmitted from person to person through contact with infected bodily fluids. This strain is named for the swollen glands, or buboes, around the groin, armpit and neck. This swelling is accompanied by fever, chills, headache and exhaustion. Symptoms occur within two or three days and typically last between one and six days. Unless treated within the first 24 hours of infection, 70 percent of those infected die [source: Chamberlain]. Pneumonic plague is less common and spreads through the air by coughs, sneezes and face-to-face contact. Its symptoms include high fever, cough, bloody mucus and difficulty breathing.
Plague victims themselves -- both dead and alive -- have historically served as effective delivery vehicles for this biological weapon. A 1940 plague epidemic occurred in China following a Japanese attack that involved dropping sacks of infected fleas out of airplanes. Today, experts predict that plague would likely be weaponized in the form of an aerosol, resulting in an outbreak of pneumonic plague. However, low-tech, vermin-based attacks are still possible.
Several countries have explored the use of plague as a bioweapon and, as the disease still occurs naturally throughout the world, copies of the bacterium are relatively easy to come by. With appropriate treatment, plague's mortality rate can dip as low as 5 percent [source: BBC]. There is no vaccine.
A bioweapon doesn't have to boast a high mortality rate to be successful, though. Consider our next entry.

Plague Hoarders
Among the countries known or suspected to have pursued plague as a biological weapon are Canada, Egypt, Germany, Japan, North Korea, Russia and the United States [source: NOVA].

Bioweapon 6: Tularemia

This image might not be as frightening as last section's plague painting, but these cute, furry bunnies could carry one of the most infectious bacteria known to man.
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While tularemia only claims an overall 5 percent mortality rate, the microorganism that causes it is one of the most infectious bacteria on Earth [source: BBC]. In 1941, the Soviet Union reported 10,000 cases of the illness. Then, during the German siege of Stalingrad the following year, this number skyrocketed to 100,000. Most of these cases occurred on the German side of the conflict. Former Soviet bioweapons researcher Ken Alibek argued that this surge in infections was no accident, but the result of biological warfare. Alibek would go on to help develop a strain of vaccine-resistant tularemia for the Soviets, before defecting to the United States in 1992.
Francisella tularensis occurs naturally in no more than 50 organisms and is especially prevalent in rodents, rabbits and hares. Humans typically acquire the disease through contact with infected animals, infected insect bites, the consumption of contaminated foods or the inhalation of the bacteria in aerosol form.
Symptoms typically appear within 3 to 5 days and vary depending on the method of infection. Patients may experience fever, chills, headache, diarrhea, muscle aches, joint pain, dry cough and progressive weakness. Pneumonialike symptoms can also develop. If untreated, respiratory failure, shock and death can follow. The illness typically lasts less than two weeks, but during that time, the infected people are basically bedridden.
Tularemia doesn't transfer between human hosts and can be easily treated with antibiotics or prevented with a vaccine. It does, however, spread very rapidly between animal hosts and humans or when used in aerosol form. It is this factor, not its mortality rate, that earned F. tularensis a Category A biological weapon ranking. It is especially virile in aerosol form. Due to these factors, the United States, Britain, Canada and the Soviet Union all worked to create weaponized tularemia after the close of World War II [source: Alibek].
If the idea of discovering bioweapons in cute little rabbits sound scary, just consider our next entry. It's all around you and you can't even see it.

Bioweapon 5: Botulinum Toxin

You don't have to watch the skies for enemy bombers to spot the threat of botulism. The spores that generate the toxin could be as close as your kitchen pantry.
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Take a deep breath. If the air you just inhaled contained botulinum toxin, you'd have no way of knowing. In weaponized airborne form, the deadly bacteria would be completely colorless and odorless. Between 12 and 36 hours later, however, the first signs of botulism would begin to take hold: blurred vision, vomiting and difficulty swallowing. At this point, your only hope would be a botulism antitoxin -- and only if you could get your hands on it before symptoms advanced much further. If untreated, paralysis begins to take hold, seizing up your muscles and finally your respiratory system.
Without respiratory support, Clostridium botulinum can kill in 24 to 72 hours. For this reason, the organism's deadly toxin rounds out the list of six Category A biological weapons. With ventilators to work your lungs, the mortality rate plummets from 70 percent to 6 percent, but recovery takes time [source: Chamberlain]. This is because the toxin binds to the point where nerve endings and muscles meet, effectively cutting off the signal from the brain. To recover fully from a case of botulism, the patient actually has to grow new nerve endings -- a process that takes several months. And while a vaccine exists, concerns over effectiveness and side effects have plagued its development, so it's not widely used.
As if the symptoms weren't scary enough, C. botulinum occurs all over the world, especially in soil and marine sediments. The spores often pop up on fruits, vegetables and seafood. In this state, they're harmless. It's only as they begin to grow that they produce their deadly toxin. Humans primarily encounter the toxin through the consumption of tainted foods, as the temperatures and chemicals in improperly stored foods often provide the perfect conditions for the spores to grow and develop. Deep wounds and infant intestinal tracks also present similar conditions.
Its power, availability and limited treatability have made botulinum toxin a favorite among several countries' bioweapons programs. Luckily, effectively using such a weapon can still provide challenges. In 1990, members of the Japanese cult Aum Shinrikyo released an aerosol of the toxin against several political targets, but were unable to cause the mass deaths they desired. When the cult switched to the chemical agent sarin gas in the 1995 attack, however, they killed a dozen people and injured thousands.
But bioweapons don't have to focus on hurting the enemy directly. As our next two entries illustrate, they can dramatically affect the food supply.

The Healing Power of Botulism
It may have an untreated mortality rate of 70 percent, but as it turns out, botulism isn't all bad. By using small amounts of purified botulinum toxin, doctors are able to harness the bacteria's paralytic power to help sooth neurologic diseases, facial ticks and even smooth away wrinkles. You probably know it better by its brand name: Botox.

Bioweapon 4: Rice Blast

Aerial dispersal of rice blast could prove disastrous to rice fields such as these in the Philippines. Khin Maung Win/AFP/Getty Images

A number of bacteria, viruses and toxins pose a significant threat to human beings, but plenty of the world's biological agents prefer different prey: cultivated food crops. Cutting off an enemy's food supply is a time-tested military strategy, whether you're defending your homeland against an invading force or besieging a walled city. Without food, populations weaken, panic, riot and eventually die.
Several countries, especially the United States and Russia, have devoted a great deal of research to diseases and even insects that target key food crops. The fact that modern agriculture typically focuses on the large-scale production of a single crop only sweetens the deal for the architects of blight and famine.
One such bioweapon is rice blast, a crop disease caused by the fungus Pyricularia oryzae (also known as Magnaporthe grisea). The leaves of affected plants soon develop grayish lesions composed of thousands of fungal spores. These spores quickly multiply and spread from plant to plant, sapping the plants and leading to much lower crop production. While breeding resistant plants is a good defensive measure against some crop disease, rice blast presents a problem because you wouldn't have to breed resistance to one strain of fungus, but 219 different strains.
Such a bioweapon wouldn't be as sure of a killer as the likes of smallpox and botulism. It could however lead to severe starvation in poorer countries, as well as financial losses and other huge problems.
A number of countries have pursued rice blast as a biological weapon, including the United States. By the time the U.S. dismantled its anti-crop program, it had amassed nearly a ton of the harmful fungus for a potential attack on Asia [source: BBC].
What's that? You prefer a nice hamburger to a rice dish? Well, our next entry proves that you meat eaters aren't safe either.

Bioweapon 3: Rinderpest

This may look like a big furry cow, but it's actually a medieval bioweapons delivery system.
Colin Monteath/Hedgehog House/Minden Pictures/Getty Images

When Genghis Khan invaded Europe in the 13th century, he inadvertently unleashed a fearsome biological weapon in the wake of his conquest. The gray steppe cattle used by his supply trains introduced a deadly cattle plague, known throughout the world today by its German name, rinderpest.
Rinderpest is caused by a virus closely related to measles, and it affects cattle and other ruminant animals such as goats, bison and giraffes. The condition is highly contagious, causing fever, loss of appetite, dysentery and inflammation of the mucus membranes. The condition drags on for six to 10 days, when the animal typically succumbs to dehydration.
Over the centuries, humans have introduced rinderpest-infected animals to various corners of the globe, often resulting in millions of dead cattle, along with other livestock and wild animals. At times, outbreaks in Africa have been so severe as to turn starving lions into man-eaters and lead ruined herdsmen to commit suicide. Thanks to extensive quarantine and vaccination programs, rinderpest has been brought under control in much of the world.
While Genghis Khan wielded rinderpest as a weapon by accident, many modern countries aren't as innocent. Canada and the United States have both researched use of the virus as an anti-livestock bioweapon [source: Scott].
Many of the scariest bioweapons out there have their roots in the ancient world. A few, however, are terrifyingly new.

Bioweapon 2: Nipah Virus

What potential biological weapons lurk in the undiscovered corners of our world?
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Viruses adapt and evolve over time. New strains emerge and, occasionally, close contact between humans and animals allow life-threatening diseases to leap to the top of the food chain. As human populations continue to swell, the emergence of new diseases is inevitable. And every time a new outbreak makes the headlines, you can be sure someone is considering how to turn it into a weapon.
Nipah virus is just such a disease, having only risen to the attention of world health agencies in 1999. The outbreak occurred in the Nipah region of Malaysia, infecting 265 and killing 105. While 90 percent of those infected handled pigs for a living, health workers suspect the virus naturally occurs in fruit bats. The exact nature of transference is uncertain, but experts think that the virus may spread through close physical contact or contaminated body fluids. Human-to-human transmission hasn't been reported yet.
The illness typically lasts 6 to 10 days, inducing symptoms that range from mild, flulike conditions such as fever and muscle pains to encephalitis, or inflammation of the brain. In these more severe cases, patients experienced drowsiness, disorientation, convulsions and ultimately coma. The virus carries a mortality rate of 50 percent, and there currently are no standard treatments or vaccinations [source: WHO].
Nipah virus, along with a number of other emerging pathogens, is classified as a Category C biological weapon. While no country is known to have researched its weaponization, its potential for widespread use and 50 percent mortality rate make it a bioweapon to watch for.
Is nature constantly coming up with new ways for us to destroy each other? Well, it's not working hard enough for some people. With our last entry, we'll look at how some scientists hope to improve on nature's existing deadly designs.

Bioweapon 1: Chimera Viruses

Modern genetic science allows us unprecedented power to manipulate organisms -- a power that, in the wrong hands, can lead to the creation of new, horrific bioweapons.
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Plague, smallpox, anthrax -- the world's deadliest biological agents aren't out to get you. Any harmful properties they possess are simply byproducts of their evolution. But what happens when scientists tinker with the genetic makeup of these organisms? What kind of horrors may come to life when we add the human desire to wage war to their natural design? Unfortunately, the creation of such life forms isn't just a page from a science fiction novel -- it's already happening.
In Greek and Roman mythology, the chimera combined elements of lion, goat and serpent into one monstrous form. Artists in the late medieval age often used the creature as a symbol to illustrate the complex nature of evil. In modern genetic science, a chimeric organism is a life form that contains genes from a foreign species. Given its namesake, you might expect all chimeric organisms to be awful examples of man twisting nature for nefarious ends. Fortunately, our increased understanding of genetic science has led to some beneficial creations. One such chimera, which combines the common cold with polio, may help cure brain cancer.
But as the war continues its forward momentum through human history, the abuse of such science is inevitable. Geneticists have already discovered the means to increase the lethality of such bioweapons as smallpox and anthrax by tweaking their genetic structure. By combining genes, however, scientists could theoretically create a virus that triggered two diseases at once. During the late 1980s, the Soviet Union's Chimera Project studied the feasibility of combining smallpox and Ebola into one super virus [source: Alibek].
Other potential nightmare scenarios involve strains of viruses that require certain triggers. A stealth virus would remain dormant for an extended period until triggered by predetermined stimuli. Other possible chimeric bioweapons might require two components to become effective. Imagine a strain of botulinum toxin that, when combined with the botulinum toxin antidote, only becomes more lethal. Such a biological attack would not only result in a higher mortality rate, but might erode public trust in health initiatives, aid workers and government response to the outbreak.
From splitting the atom to cracking life's genomic riddles, the last century of scientific research has brought about tremendous potential for humans to build a better world -- or destroy the one they have.