Very few humans have gone up against bird flu. But we've all dealt with seasonal flu for years. Some of our immune systems might be primed to fend off a worse case, research finds.

H7N9 — is detected in at a poultry farm; scientists learn past human flu exposure may provide some immunity to H5N1.

The health secretary has suggested allowing the virus to spread, so as to identify birds that may be immune. Such an experiment would be disastrous, scientists say.