How does a new virus develop?
When the genes that govern the H and N proteins reproduce, random changes _ mutations _ can occur in their DNA. The changes gradually accumulate, ultimately producing a virus that may be more lethal or may penetrate a target cell more easily. Another possibility is gene-swapping. This can happen when a cell is infected by viruses from different creatures, say a chicken and a pig. The cell becomes a “mixing bowl,” whipping up a new virus containing some chicken genes and some pig genes. In the new strain of H1N1 virus, pieces of human, bird and pig genes are all scrambled up.
