It could have been caused by a single spore which has increased by geometrical progression.

But some experts contend that just a few spores, maybe even a single spore, could infect a susceptible person in rare circumstances.

In his view, a single spore has a chance, however small, of infecting someone.

Spores are the units of asexual reproduction, because a single spore develops into a new organism.

Indeed, at least one germ warfare expert has suggested that a single spore might do it.

One kind, the megasporangium, produces only a single large spore (a megaspore).

Not a single spore left in sight.

"We don't want to lose a single spore of anthrax," a government investigator said.

They never found a single spore or any other clue to how Ms. Nguyen became infected.

But these are just the tips of a living iceberg spawned ages ago by a single spore.