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A bioterrorism attack is the deliberate release of viruses, bacteria, or other germs (agents) used to cause illness or death in people, animals, or plants. These agents are typically found in nature, but it is possible that they could be changed to increase their ability to cause disease, make them resistant to current medicines, or to increase their ability to be spread into the environment. Biological agents can be spread through the air, through water, or in food. Terrorists may use biological agents because they can be extremely difficult to detect and do not cause illness for several hours to several days. Some bioterrorism agents, like the smallpox virus, can be spread from person to person and some, like anthrax, can not.

Bioterrorism Agent Categories

Bioterrorism agents can be separated into three categories, depending on how easily they can be spread and the severity of illness or death they cause. Category A agents are considered the highest risk and Category C agents are those that are considered emerging threats for disease.

  • Category A 

    These high-priority agents include organisms or toxins that pose the highest risk to the public and national security because:

    • They can be easily spread or transmitted from person to person
    • They result in high death rates and have the potential for major public health impact
    • They might cause public panic and social disruption
    • They require special action for public health preparedness.
      • Anthrax (Bacillus anthracis)

      • Botulism (Clostridium botulinum toxin)

      • Plague (Yersinia pestis)

      • Smallpox(variola major)

      • Tularemia(Francisella tularensis)

      • Viral hemorrhagic fevers(filoviruses [e.g., Ebola, Marburg] and arenaviruses [e.g., Lassa, Machupo])

  • Category B 

    These agents are the second highest priority because:

    • They are moderately easy to spread
    • They result in moderate illness rates and low death rates
    • They require specific enhancements of CDC's laboratory capacity and enhanced disease monitoring.
      • Brucellosis (Brucella species)

      • Epsilon toxin of Clostridium perfringens

      • Food safety threats (e.g., Salmonella species, Escherichia coli O157:H7, Shigella)

      • Glanders (Burkholderia mallei)

      • Melioidosis(Burkholderia pseudomallei)

      • Psittacosis (Chlamydia psittaci)

      • Q fever (Coxiella burnetii)

      • Ricin toxin from Ricinus communis (castor beans)

      • Staphylococcal enterotoxin B

      • Typhus fever (Rickettsia prowazekii)

      • Viral encephalitis (alphaviruses [e.g., Venezuelan equine encephalitis, eastern equine encephalitis, western equine encephalitis])

      • Water safety threats (e.g., Vibrio cholerae, Cryptosporidium parvum)

  • Category C 

    These third highest priority agents include emerging pathogens that could be engineered for mass spread in the future because:

    • They are easily available
    • They are easily produced and spread
    • They have potential for high morbidity and mortality rates and major health impact.
      • Emerging infectious diseases such as Nipah virus and hantavirus


Scientists are most concerned about smallpox and anthrax. Both are bacteria that can spread through the air in a powder and cause swift, deadly diseases. Smallpox could be even more lethal because it's easily spread from one person to another. Also worrisome are the bubonic plague, botulism, tularemia and ebola.

There is no real preventative measures other than stopping the initial introduction of the bio-terror substance. It would  not be apparent that a biological agent has been dispersed until people begin falling ill several days later. For most biological agents, the initial symptoms would resemble a flu-like malaise.  



Anthrax is an acute infectious disease caused by the spore-forming bacterium Bacillus anthracis. Anthrax most commonly occurs in wild and domestic lower vertebrates (cattle, sheep, goats, camels, antelopes, and other herbivores), but it can also occur in humans when they are exposed to infected animals or tissue from infected animals

Given appropriate weather and wind conditions, 50 kilograms of anthrax released from an aircraft along a 2 kilometer line could create a lethal cloud of anthrax spores that would extend beyond 20 kilometers downwind. The aerosol cloud would be colorless, odorless and invisible following its release. Given the small size of the spores, people indoors would receive the same amount of exposure as people on the street.

There are currently no atmospheric warning systems to detect an aerosol cloud of anthrax spores. The first sign of a bioterrorist attack would most likely be patients presenting with symptoms of inhalation anthrax.

Symptoms of disease vary depending on how the disease was contracted, but symptoms usually occur within 7 days.

Cutaneous: Most (about 95%) anthrax infections occur when the bacterium enters a cut or abrasion on the skin, such as when handling contaminated wool, hides, leather or hair products (especially goat hair) of infected animals. Skin infection begins as a raised itchy bump that resembles an insect bite but within 1-2 days develops into a vesicle and then a painless ulcer, usually 1-3 cm in diameter, with a characteristic black necrotic (dying) area in the center. Lymph glands in the adjacent area may swell. About 20% of untreated cases of cutaneous anthrax will result in death. Deaths are rare with appropriate antimicrobial therapy.

Inhalation: Initial symptoms may resemble a common cold. After several days, the symptoms may progress to severe breathing problems and shock. Inhalation anthrax is usually fatal.

Intestinal: The intestinal disease form of anthrax may follow the consumption of contaminated meat and is characterized by an acute inflammation of the intestinal tract. Initial signs of nausea, loss of appetite, vomiting, fever are followed by abdominal pain, vomiting of blood, and severe diarrhea. Intestinal anthrax results in death in 25% to 60% of cases.

A 1970 analysis by the World Health Organization concluded that the release of aerosolized anthrax upwind of a population of 5,000,000 could lead to an estimated 250,000 casualties, of whom as many as 100,000 could be expected to die.

A later analysis, by the Office of Technology Assessment of the U.S. Congress, estimated that 130,000 to 3 million deaths could occur following the release of 100 kilograms of aerosolized anthrax over Washington D.C., making such an attack as lethal as a hydrogen bomb.

Vaccine supplies are limited and U.S. production capacity is modest. There is no vaccine available for civilian use. Incubation (the time from exposure to symptoms) 2-60 days.


The disease is at least 3000 years old. Smallpox is caused by the virus variola. Smallpox is a poxvirus, characterized by a brick-shape, containing linear double stranded DNA, a disk-shaped core within a double membrane, and a lipoprotein envelope. The virion contains a DNA-dependant RNA polymerase. This enzyme is required because the virus replicates in the cytoplasm and does not have access to the cellular RNA polymerase, which is located in the nucleus.

An aerosol release of smallpox virus would disseminate readily given its considerable stability in aerosol form and epidemiological evidence suggesting the infectious dose is very small. Even as few as 50-100 cases would likely generate widespread concern or panic and a need to invoke large-scale, perhaps national emergency control measures.

Some of the reasons bioterrorists prefer smallpox are its high fatality rates – it kills some 30 percent of its victims – and its long incubation periods – up to 14 days. Symptoms: There may be no symptoms for first two weeks after infection. About two weeks after infection, the victim may develop high fever, malaise, headache and backache. Two days after symptoms start, a rash develops, spreading all over the body. 

Several factors fuel the concern: the disease has historically been feared as one of the most serious of all pestilential diseases; it is physically disfiguring; it bears a 30 percent case-fatality rate; there is no treatment; it is communicable from person to person; and no one in the U.S. has been vaccinated during the past 25 years.

The United States currently has a limited supply of smallpox vaccine (approximately 15 million doses) available for emergency use, if needed. New methods for the production of additional smallpox vaccine in large quantities are being explored. At this time, no preventive vaccination program is planned.


Ebola hemorrhagic fever (Ebola HF) is a severe, often-fatal disease in humans and nonhuman primates (monkeys and chimpanzees) that has appeared sporadically since its initial recognition in 1976.

The disease is caused by infection with Ebola virus, named after a river in the Democratic Republic of the Congo (formerly Zaire) in Africa, where it was first recognized. The virus is one of two members of a family of RNA viruses called the Filoviridae. Three of the four species of Ebola virus identified so far have caused disease in humans: Ebola-Zaire, Ebola-Sudan, and Ebola-Ivory Coast. The fourth, Ebola-Reston, has caused disease in nonhuman primates, but not in humans. 

Infection with Ebola virus in humans is incidental -- humans do not "carry" the virus. Because the natural reservoir of the virus is unknown, the manner in which the virus first appears in a human at the start of an outbreak has not been determined. However, researchers have hypothesized that the first patient becomes infected through contact with an infected animal. 

After the first case-patient in an outbreak setting (often called the index case) is infected, humans can transmit the virus in several ways. People can be exposed to Ebola virus from direct contact with the blood and/or secretions of an infected person. This is why the virus has often been spread through the families and friends of infected persons: in the course of feeding, holding, or otherwise caring for them, family members and friends would come into close contact with such secretions. People can also be exposed to Ebola virus through contact with objects, such as needles, that have been contaminated with infected.


Plague, caused by a bacterium called Yersinia pestis, is transmitted from rodent to rodent by infected fleas.

Plague is characterized by periodic disease outbreaks in rodent populations, some of which have a high death rate. During these outbreaks, hungry infected fleas that have lost their normal hosts seek other sources of blood, thus increasing the increased risk to humans and other animals frequenting the area.

Plague is transmitted from animal to animal and from animal to human by the bites of infective fleas. Less frequently, the organism enters through a break in the skin by direct contact with tissue or body fluids of a plague-infected animal, for instance, in the process of skinning a rabbit or other animal. Plague is also transmitted by inhaling infected droplets expelled by coughing, by a person or animal, especially domestic cats, with pneumonic plague. Transmission of plague from person to person is uncommon and has not been observed in the United States since 1924 but does occur as an important factor in plague epidemics in some developing countries.

Epidemics of plague in humans usually involve house rats and their fleas. Rat-borne epidemics continue to occur in some developing countries, particularly in rural areas. The last rat-borne epidemic in the United States occurred in Los Angeles in 1924-25. Since then, all human plague cases in the U.S. have been sporadic cases acquired from wild rodents or their fleas or from direct contact with plague-infected animals.

Rock squirrels and their fleas are the most frequent sources of human infection in the southwestern states. For the Pacific states, the California ground squirrel and its fleas are the most common source. Many other rodent species, for instance, prairie dogs, wood rats, chipmunks, and other ground squirrels and their fleas, suffer plague outbreaks and some of these occasionally serve as sources of human infection. Deer mice and voles are thought to maintain the disease in animal populations but are less important as sources of human infection. Other less frequent sources of infection include wild rabbits, and wild carnivores that pick up their infections from wild rodent outbreaks. Domestic cats (and sometimes dogs) are readily infected by fleas or from eating infected wild rodents. Cats may serve as a source of infection to persons exposed to them. Pets may also bring plague-infected fleas into the home.


Botulism is a rare but serious paralytic illness caused by a nerve toxin that is produced by the bacterium Clostridium botulinum. There are three main kinds of botulism. Foodborne botulism is caused by eating foods that contain the botulism toxin. Wound botulism is caused by toxin produced from a wound infected with Clostridium botulinum. Infant botulism is caused by consuming the spores of the botulinum bacteria, which then grow in the intestines and release toxin. All forms of botulism can be fatal and are considered medical emergencies. Foodborne botulism can be especially dangerous because many people can be poisoned by eating a contaminated food.

The classic symptoms of botulism include double vision, blurred vision, drooping eyelids, slurred speech, difficulty swallowing, dry mouth, and muscle weakness. Infants with botulism appear lethargic, feed poorly, are constipated, and have a weak cry and poor muscle tone. These are all symptoms of the muscle paralysis caused by the bacterial toxin. If untreated, these symptoms may progress to cause paralysis of the arms, legs, trunk and respiratory muscles. In foodborne botulism, symptoms generally begin 18 to 36 hours after eating a contaminated food, but they can occur as early as 6 hours or as late as 10 days.


Tularemia, a disease that can affect both animals and humans, is caused by a bacteria, Francisella tularemsis. Although many wild animals are infected, (hares, rabbits, squirrels, muskrats, beavers, deer), occasionally certain domestic animals can be infected (sheep and cats). The rabbit is the species most often involved in disease outbreaks. The bacteria can also be found in ticks and deerflies.

Many routes of human exposure to the tularemia bacteria are known to exist. The common routes include inoculation of the skin or mucous membranes with blood or tissue while handling infected animals, the bite of an infected tick, contact with fluids from infected deer flies or ticks, or handling or eating insufficiently cooked rabbit meat. Less common means of spread are drinking contaminated water, inhaling dust from contaminated soil or handling contaminated pelts or paws of animals. Tularemia is not spread from person to person.

Tularemia is usually recognized by the presence of a skin lesion and swollen glands. Ingestion of the organism may produce a throat infection, intestinal pain, diarrhea and vomiting. Inhalation of the organism may produce a fever alone or combined with a pneumonia-like illness.


Credit: CDC, The FBI