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The presence of the virus, as well as the husks of cells left by the replication process, alerts the immune system that something is wrong. The body sends out an alarm in the form of molecules called cytokines and recruits attackers called T-cells to kill infected cells before they release their toxic cargo.
These defenses come with a cost.
Most of the symptoms we attribute to a flu virus are actually the result of the body's defensive maneuvers. For example, fever occurs when cytokines tell the brain to raise the body's temperature, which helps the immune system fight its enemies.
"Part of the disease is the damage the virus causes and part of it is the body's response to the virus," Alexander said, comparing the process to firefighters extinguishing a fire and causing water damage in the process.
The very young, elderly or sick may have a weak immune system that is unable to keep up with the virus or allows a second infection to take root, sometimes resulting in death.
More than 30,000 Americans a year die from influenza-related illness, according to the Centers for Disease Control and Prevention.
Especially alarming to public health officials are flu strains that kill young adults, as was seen in 1918 and is being reported from Mexico now. In these cases,
a person's healthy immune system may overreact to the virus, causing a "cytokine storm" that can cause excessive inflammation in a patient's lungs, leading to death.
"It is almost as though the
patient self-destructed," Alexander said.
http://www.venturacountystar.com/news/2009/may/01/virus-invasion-sets-battle-inside-body/