| 1 | That provides ample opportunity for strains to mingle and recombine. | |
| 2 | This particular H1N1 strain has not circulated previously in humans. | |
| 3 | The H1N1 virus itself grows more efficiently than some past flu strains. | |
| 4 | Preparation of the vaccine strain (called vaccine virus): The virus must first be adapted for use in manufacturing vaccine. | |
| 5 | Scientists have found how some strains of the malaria parasite resist treatment, a discovery that could lead to a new generation of antimalarial drugs. |