Frequent subsequent waves of the plague continued to strike throughout the 6th, 7th and 8th centuries, with the disease becoming more localized and less virulent.

Most of the fatalities are caused not by foreign diseases but by familiar pathogens that have become more virulent.

There is no evidence that any of the numerous other vaccinia recombinants have become more virulent.

The stench grew stronger as I drew nearer the lake, becoming more fierce and virulent with every step.

Dr. Gubler said scientists also needed to investigate whether the deaths of the birds indicated that West Nile virus had become more virulent.

As Heather matured, their riffs had become more frequent and virulent.

"At the moment not surprisingly, when we have begun to investigate politics, the attack has become virulent," said a deputy prosecutor, Guido Lo Forte.

They have become less virulent than the strains prevailing in Bangladesh.

E. histolytica also responds to the presence of serotonin by becoming more virulent.

It is believed that HIV arose from another, less harmful virus, that mutated and became more virulent.