It might be a common cold, or it could be the start of a vast pandemic — most likely due to a scientific experiment gone wrong — that will wipe out most of humanity.

Most over-65s are protected, because the strain is similar to one that circulated years ago, so it is a younger age group, including children and pregnant women, who are being hardest hit.

Seasonal influenza comes and goes (typically, it affects up to 20% of America’s population), with the virus evolving slightly from one winter to the next.