Later symptoms include cough, chest discomfort, shortness of breath, fatigue and muscle aches. The CDC warns to not assume a person has inhalation anthrax if he or she displays cold or flu symptoms.

Symptoms of anthrax infection usually appear within seven days of coming in contact with the bacterium, but in cases of inhalation anthrax, symptoms appear anywhere from one week to 42 days after exposure.

Of the three anthrax forms, inhalation anthrax is the most lethal when untreated. Even with antibiotics, the mortality rate is 75 percent.

The survival rate of cutaneous anthrax - the least deadly form - is 80 percent without treatment and 99 percent with treatment. In cases of gastrointestinal anthrax, at least 25 percent of untreated persons will die.

Antibiotics and vaccines

All forms of anthrax are treated with antibiotics, though the course of treatment depends on whether the person is already sick.

If a person has been exposed to anthrax but hasn't become ill, doctors use antibiotics in conjunction with the anthrax vaccine to prevent infection.

Available antibiotics include ciprofloxacin and doxycycline. If a person becomes infected, doctors typically prescribe a 60-day antibiotic treatment.

Anthrax can be prevented with the licensed vaccine for anthrax, Anthrax Vaccine Adsorbed.

AVA is currently given to prevent infection only to people at risk of anthrax exposure, including people in the military and laboratory workers.

The public may receive the vaccine if another anthrax attack occurs, the CDC said.

Scientists who work with anthrax at Fort Detrick's U.S. Army Medical Research Institute of Infectious Diseases receive the vaccine, according to USAMRIID spokeswoman Caree Vander Linden.

Lt. Col. Ross Pastel, USAMRIID's deputy commander for safety, biosurety, operations, plans and security, said it is possible for scientists who work with anthrax to inhale the bacteria without becoming ill. Scientists working with anthrax must be vaccinated, and one's susceptibility to the agent varies, he said.

There is a difference between what is known as LD50, which means a lethal dose for 50 percent of the population, and what an individual's tolerance is, he said.

Anthrax as a weapon

The CDC classifies agents of bioterrorism into three priority areas - A, B and C. Anthrax is classified as a Category A agent, which means:

Other Category A agents include botulism, smallpox and ebola.

There are no known cases of anthrax transmission from person to person, according to the CDC.

Though anthrax is naturally occurring, the agent has been used as a weapon, beginning in a 1930s Japanese biological warfare program in Manchuria.

In a refined form, anthrax is highly infectious, floating through air easily, causing inhalation anthrax. The agent is extremely stable, able to survive in an environment for decades.

In fall 2001, refined anthrax spores - much more infectious than naturally occurring spores - were mailed in letters to U.S. Sens. Tom Daschle (D-S.D.) and Patrick Leahy (D-Vt.), as well as to media outlets in New York and Florida.

The attacks resulted in five deaths from inhalation anthrax and 17 cases of anthrax infection, including postal workers. Between 1989 and the 2001 anthrax attacks, only one case of anthrax was reported due to naturally occurring spores, according to the CDC.