The current epidemic strain evolved at the beginning of the 20th century - probably between 1908 and 1943.

More than 100 types of cold viruses are known, and new strains of flu evolve every few years.

Nevertheless, new strains may evolve that escape immunity induced by vaccines; for example an updated influenza vaccine is needed each year.

If the virus is not reproducing, he explained, it cannot mutate, and drug resistant strains cannot evolve.

The effectiveness of chloroquine against P. falciparum has declined as resistant strains of the parasite evolved.

As climate, diseases, and pests change, the strain evolves and selects new defenses, programmed into the ge- netic orders contained in each generation of seeds.

Just as bacterial evolution had allowed microbes to develop resistance to penicillin, strains of S. aureus evolved to become resistant to methicillin.

Extremely virulent strains can eventually evolve by mutation and natural selection within the virus population inside a host.

The anthropotropic strains (genotype 1 and 2) have evolved more recently than the others suggesting that this virus was originally a zooenosis.

Two monkeys died six days after becoming infected, in contrast to the usual eight months or more of survival with the virus from which the fast-acting strain evolved.