influenza

Influenza, or flu, feels like a cold that has been turned up sharply: instead of a gradual sniffle it tends to hit suddenly, flattening a person with high fever, aching muscles, headache, exhaustion and a dry cough. People often say they feel as if they have been hit by a truck — that whole-body wallop is what sets flu apart from the milder common cold.

It is a contagious illness caused by influenza viruses, mainly types A and B, which infect the respiratory tract and spread through coughs, sneezes and contaminated surfaces. The virus changes its surface proteins from year to year, which is why immunity from a past infection or vaccine fades and a fresh flu vaccine is offered each season. Outbreaks sweep through communities in seasonal waves, mostly in colder months.

Most healthy people recover within a week or two with rest and fluids, but flu can be dangerous for the very young, the elderly, pregnant women and those with heart or lung disease, in whom it may lead to viral pneumonia or a secondary bacterial pneumonia. Antiviral drugs such as oseltamivir can shorten illness and reduce complications if started early, and annual vaccination remains the most effective preventive measure.

It is a common misuse of language to call any cold the flu; true influenza is a distinct and generally more severe infection, and the distinction matters when judging risk and deciding on antiviral treatment.