The flu is a respiratory illness caused by the influenza virus. The flu is not the same as a cold, although they share many of the same symptoms. The cold is caused by a different virus, and it tends to have milder symptoms than the flu. Colds are also less likely to cause serious complications.
When the influenza virus gets into the body, it moves into the respiratory tract. Once there, it binds to the surface of cells. The virus then opens and releases its genetic information (RNA) into the cell's nucleus. The nucleus is where the cell's genetic information (DNA and RNA) is stored. The virus replicates, or copies itself, and takes over the functions of the cell. The copies of the virus move to the cell membrane until the cell finally dies and releases them out into the body, where they go on to infect other cells.
The respiratory tissues swell up and become inflamed (the inflammation usually heals within a few weeks). As the virus moves through the respiratory tract and into the bloodstream, the first symptoms begin to emerge. The replication process continues for up to several days, until the body's immune system begins to fight the virus off.
Flu symptoms can include any or all of the following:
- Body aches
- Runny nose and congestion
These symptoms, although uncomfortable, are generally not dangerous. But the flu also weakens the immune system, leaving it vulnerable to more serious infections. High-risk individuals (see Who Is at Risk?) in particular are susceptible to serious complications, such as:
- Bacterial pneumonia
- Sinus problems and ear infections (primarily in children)
- Worsening of preexisting conditions, such as asthma or diabetes