One of the ways it can spread to healthy poultry farms is by a single infected duck visiting a flock and shedding the virus through feces. H5N1 kills relatively quickly, causing respiratory distress, swelling, lack of coordination and other symptoms before death. (Culling is therefore considered more humane.)

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.