The source of infection for this case has not been identified.

This is because the things that the individual touches normally become sources of infection.

The investigation is also looking into other sources of infection.

Ground beef is therefore much more likely to be a source of infection than steak.

"The hospital has been the main source of infection," she said.

However, in some patients, no other source of infection is found.

But water is not the only possible source of infection.

That letter had been considered the likely source of infection for all the anthrax cases in and around Washington.

The source of infection in these individuals has not been determined.

It is often difficult even to determine the source of infection.