Why do pandemic flu's target healthy young people?

Why are healthy adults at greatest risk? It appears that the H5N1 bird-flu virus causes a massive immunological response against the virus in those with the strongest immune systems. Unfortunately, this causes the release of human enzymes called "cytokines," which destroy lung cells along with viral particles. This, in turn, causes a deadly outpouring of fluids into the lung, which interferes with the exchange of oxygen and carbon dioxide. This reaction is frequently fatal. There are few effective treatments--one of the only treatments available is to place the patient on a ventilator, a mechanical respirator. Even with this treatment, patients often die from complications.

The question that we all have today is whether the current H5N1 influenza virus, often referred to as the bird flu virus, is the likely source of the next pandemic. The most honest answer that we as scientists can tell you is that we don't know. However, having said that, there are many ominous signs that indicate that it very well could be the source for the next pandemic and could begin at any time. The 1957 and 1968 pandemics occurred as the result of an avian influenza virus combining with a human influenza virus to form a third influenza virus capable of infecting humans. This process is known as reassortment. Until recently, we believed that this was the most likely way for a new influenza virus that can be readily transmitted between humans to be created. However, research done by Dr. Jeffrey Taubenberger of the Armed Forces Institute of Pathology and other colleagues have clearly demonstrated that the 1918 virus jumped right from birds into humans as a readily human-to-human transmitted infectious agent. Today we are very concerned that the H5N1 virus circulating in wild and domesticated birds in Asia is moving genetically towards a human-to-human transmitted agent and could result in a 1918-like pandemic. If this happens, we expect that the illnesses that we see among those infected with H5N1 virus will be similar to those individuals who were infected in 1918 and the limited number of humans who have been infected with H5N1 to date. Many of the deaths that occurred in these populations resulted from the explosive growth of the virus in humans and a subsequent cytokine storm. A cytokine storm is the release of a chemical in the body that stimulates the human immune system to respond to the virus infection. In these serious illnesses and deaths, it's actually been an over vigorous immune response elicited by this infection that result in the organ damage and ultimately the death of the individual. Ironically this means that those with the strongest immune systems may be at highest risk for a serious outcome if infected with the H5N1 virus. At the same time, it is surely possible that those with weakened or immature immune systems, such as the very young or very old, and those with underlying immune conditions, may experience serious illness associated with the annual influenza illness, which often involves damage to the respiratory tract and subsequent secondary bacterial infection.

Here we go:

Quote:

Why are healthy adults at greatest risk? It appears that the H5N1 bird-flu virus causes a massive immunological response against the virus in those with the strongest immune systems. Unfortunately, this causes the release of human enzymes called "cytokines," which destroy lung cells along with viral particles. This, in turn, causes a deadly outpouring of fluids into the lung, which interferes with the exchange of oxygen and carbon dioxide. This reaction is frequently fatal. There are few effective treatments--one of the only treatments available is to place the patient on a ventilator, a mechanical respirator. Even with this treatment, patients often die from complications.

Oh, and I dodn't think this sort of flu TARGETS young healthy people...it just KILLS 'em more than you'd expect.

When the immune system is fighting pathogens, cytokines signal immune cells such as T-cells and macrophages to travel to the site of infection. In addition, cytokines activate those cells, stimulating them to produce more cytokines. Normally, this feedback loop is kept in check by the body. However, in some instances, the reaction becomes uncontrolled, and too many immune cells are activated in a single place. The precise reason for this is not entirely understood but may be caused by an exaggerated response when the immune system encounters a new and highly pathogenic invader. Cytokine storms have potential to do significant damage to body tissues and organs.[citation needed] If a cytokine storm occurs in the lungs, for example, fluids and immune cells such as macrophages may accumulate and eventually block off the airways, potentially resulting in death.[citation needed]

The cytokine storm (hypercytokinemia) is the systemic expression of a healthy and vigorous immune system resulting in the release of more than 150 inflammatory mediators (cytokines, oxygen free radicals, and coagulation factors).[citation needed] Both pro-inflammatory cytokines (such as Tumor necrosis factor-alpha, Interleukin-1, and Interleukin-6) and anti-inflammatory cytokines (such as interleukin 10 and interleukin 1 receptor antagonist) are elevated in the serum of patients experiencing a cytokine storm.[citation needed]

Cytokine storms can occur in a number of infectious and non-infectious diseases including graft versus host disease (GVHD), adult respiratory distress syndrome (ARDS), sepsis, avian influenza, smallpox, and systemic inflammatory response syndrome (SIRS).[2]

The first reference to the term cytokine storm in the published medical literature appears to be by Ferrara et al.[3] in GVHD in February 1993.


[edit] Role in pandemic deaths
It is believed that cytokine storms were responsible for many of the deaths during the 1918 influenza pandemic, which killed a disproportionate number of young adults.[1] In this case, a healthy immune system may have been a liability rather than an asset. Preliminary research results from Hong Kong also indicated this as the probable reason for many deaths during the SARS epidemic in 2003.[citation needed] Human deaths from the bird flu H5N1 usually involve cytokine storms as well.[

Was there an eventual strategy employed by the Spanish Flu to evolve into aless virulent strain?

"With infections in many different communities as we're seeing, we don't think that containment is feasible," said Anne Schuchat of the US Centers for Disease Control and Prevention (CDC).