But the genesis of a new, highly aggressive mutated strain concentrating on the Great Lakes is a biological mystery.

Beekeepers can greatly reduce this problem by killing the queens of aggressive strains to breed gentler stock.

The African bees mixed with indigenous bees to create a more aggressive strain.

Lorraine was unable to attend the wedding due to an aggressive strain of herpes that kept her from flying.

The particularly dark and aggressive strain of rock called heavy metal has been around for more than three decades.

The influenza virus rapidly rearranges its gene structure and can produce aggressive strains for which humans have few defenses.

The disease was mild for a number of years until it either mutated or a more aggressive strain was introduced around 1950.

Mr. Stern, a guitarist leading his own group, explores a rhythmically aggressive strain of jazz-rock.

Experts fear that the aggressive strains will displace more benign strains.

The new strains, more dangerous and aggressive than the old strain, spread rapidly, either carried by the wind or in seed potatoes, he said.