Critics had said the ring approach, while useful in battling natural outbreaks, would do little or nothing against a moderately skilled enemy intent on mayhem.

Weapons experts said such natural outbreaks were a main way that a terrorist might obtain starter germs, or seed stock.

In the Soviet Union the outbreak at Stalingrad was described as a natural outbreak.

In some cases, the microbes that might be used as biological agents have natural outbreaks, giving an opportunity for drugs to be tested.

But for many other agents natural outbreaks might never occur.

That approach has worked well in containing natural outbreaks in the past.

"But every indicator I'm aware of points to a natural outbreak."

Intelligence agencies believe that Iraq may have collected the smallpox virus from a natural outbreak that struck there in 1971 and 1972.

But Iraq could have developed its own supply of the virus from a natural outbreak that struck there in 1971 and 1972, infecting at least 800 people.

In 1970, a rare natural outbreak caused by airborne transmission of smallpox virus occurred when a German electrician returned from Pakistan.