In the First Few Hours
Every time a sick person coughs, she expels some 2,000 virus-laden droplets. If you inhaled any of those (or touched a door handle coated with them, and then rubbed your nose or eyes), they have floated through your nostrils and are burrowing into your airway's cells. Unlike colds, which attack the nose and throat, the flu virus can travel into the lungs.
Your immune system goes on the offensive immediately. It begins churning out antibody and T cell soldiers that latch on to and destroy the virus. Had you been vaccinated against this particular strain, you'd have antibodies stockpiled. (Still on the fence about vaccination? Why You Should Get the Flu Shot.)
For the Next Day or Two
You're unaware that the virus is now using your body's cellular machinery as a copy machine. Swollen with rapidly multiplying flu, the infected cells in your body begin bursting like water balloons, spewing virus everywhere.
You are now contagious. Hence, you should always sneeze into a tissue or cough into your inner elbow.
By Day Four
Your immune system can't keep up with the furious flu spread. Symptoms hit you like a freight train. You can go from feeling peppy at lunchtime to being bedridden by 5 p.m., beset with fever, chills, headache, and crazy-tender muscles. (Taking a few ibuprofen can help ease the aches.)
It's not the virus that makes you feel like crap. The misery stems from inflammation, the result of an immune system in "code red!" All of your body's energy is being used to slay the flu; you can hardly muster the energy to walk to the bathroom. (Once you recover, upgrade your diet with these 8 Energy- and Health-Boosting Superfoods.)
To keep dead cell debris from clogging up your lungs, you develop a dry cough. Your throat starts to ache from the irritation, which can trigger a release of mucus. A saline nasal flush might help.
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