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

2.   But the lack of definitive scientific data has left others fearful that a single spore might indeed be deadly.

3.   However, a single spore that lands on an open wound is also reason for concern.

4.   In each case, tests showed a single spore.

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

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

7.   Several infectious disease experts say, however, that no building can be sterilized or treated in a way that removes every single spore.

8.   The contamination, though limited to perhaps a single spore, also reinforced the theory that anthrax spores could leave one letter and catch a ride on another.

9.   They also say it is impossible to sterilize every single spore in a large space.

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