This 1975 microscope image made available by the Centers for Disease Control and Prevention shows a group of smallpox viruses. (Photo: Fred Murphy, AP)

The Food and Drug Administration approved the first drug to treat smallpox, a response to fears of experts that the virus could be used as a biological weapon.

TPOXX, or tecovirimat, is taken orally to treat smallpox "to lessen the impact of a potential epidemic" SIGA Technologies said in a statement

that the World Organization of Health eradicated smallpox in 1980 and the vaccination of Americans against the virus was stopped, according to the Center for Control and Prevention of Diseases

. "This new treatment offers us an additional option if smallpox is to be used as a biological weapon," said FDA Commissioner Scott Gottlieb, MD. In a statement

there is "no immediate and direct threat of a bioterrorist attack on smallpox," according to the CDC.

Researchers tested the effectiveness of the smallpox drug by studies involving animals infected with the virus. The drug has also been tested with human volunteers uninfected with smallpox.

Headache, nausea and abdominal pain remained the commonly reported side effects.

The virus has spread widely through direct contact with people, the FDA said. Symptoms usually appear 10 to 14 days after the infection. They included fever, exhaustion, headache and backache, as well as a rash initially consisting of small pink bumps progressing to pus-filled wounds.

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