In the laboratory, he identified the most common lethal pneumococcal strains, or serotypes.

Another theory holds that the 1918 virus mutated extremely rapidly to a less lethal strain.

These are generally referred to as the "lethal" and "non-lethal" strains of the species.

There are other (more common) lethal infectious strains.

To prevent more lethal strains of the disease from spreading, a country must ensure an uninterrupted supply of drugs.

More lethal strains that resist several drugs at once could easily be incubated in prison, then transmitted into the city as a whole.

If so, can scientists determine what components of the virus account for the difference between mild and lethal strains?

It was possible, they believed, that the nonçathogenic strain would overpower the lethal strain.

If a very lethal strain emerged in the Arctic, it would probably die out there.

Now the virus appears to have altered and birds appear to be living with this more lethal strain.