Arestvyr is not approved by the Food and Drug Administration except for use in emergencies.

It has never been tested on smallpox in humans because the disease was eradicated. However, it has prevented death in dozens of monkeys injected with what would normally have been lethal doses of smallpox or a related virus, monkey pox.

It also appears to have helped several humans suffering from potentially lethal reactions to smallpox vaccine, which is itself a live smallpox-related virus but is normally harmless. They included a child near death after catching his father’s vaccination virus, a soldier vaccinated just before discovering he had leukemia, and a woman whose immune system was suppressed by steroids and who was infected by touching bait meant for raccoons that contained a combined rabies/smallpox vaccine.

However, those patients were also given immune globulin, other drugs and hospital care, so it is hard to know exactly what worked.

Bioterrorism experts say the need for Arestvyr has declined since the United States increased its stockpile of smallpox vaccine, which was once given to people routinely before the disease was brought under control, including a less potent but less risky backup vaccine for those who cannot tolerate the standard one.

The word “smallpox” still strikes fear. John Grabenstein, a retired colonel and a top biodefense adviser to the Defense Department after the 2001 attacks, recalled reports of refrigerated Soviet warheads loaded with the virus that could, in theory, aerosolize it over large areas. Others have envisioned a few infected terrorists mingling in crowds.

Left untreated, smallpox kills a third of victims. But prominent experts say the danger is overblown. Because it can take up to two weeks before an infected person becomes seriously ill, and up to five more days before he or she begins to infect others, there is time to respond, they said.

Also, they said, by the time smallpox victims reach the infectious stage, when their pox are erupting, they are too sick to wander around. That is why outbreaks in schools or factories were nearly unheard of.