The influenza vurus is a tiny, redoubtable foe that survives by stealth and sheer numbers. Seen through an electron microscope, it resembles a spiky ball. It measures around 100 nanometres across, which is about a thousand times smaller than a bacterium.
Because it is so minute, the virus is unable to carry around the enzyme toolkit that it needs to reproduce. Instead, it hijacks the machinery of cells in the throat, nose and lungs to do this. It first enters the nose or mouth, inhaled in droplets expelled by a cough or a sneeze by an infected person.
A virus can also survive on surfaces for up to 72 hours, depending on the type of surface, temperature and humidity. This means the virus can be picked up on the fingers and transferred to the mucous membranes if the person touches the nose, eyes or the mouth. Cells that are attacked in the throat, lungs and muscles give rise to soar throat, respiratory wheeze and muscle ache.