Hospital officials were quoted originally as saying it was an especially virulent strain of bacteria.

But for reasons unexplained, the sample was mixed with a virulent strain.

They thought it was an extremely virulent strain of the flu.

And perhaps there's a more virulent strain of the gaffkemia bacteria out there.

"The probability of going out in nature and getting a virulent strain on the first try would be about 50-50," he said.

Finally, the research team needed to acquire a virulent strain.

Some officials said a more virulent strain of syphilis might be spreading.

It swept through us that fall like a virulent strain of influenza.

Symptoms in animals vary, but virulent strains can cause death within a few days.

We are facing an extremely virulent strain and the environmental conditions lead us to fear the worst.