As an expert in the field of epidemiology and infectious diseases, I can provide you with a comprehensive answer to your question about the possibility of getting the H1N1 flu, also known as the swine flu, twice. It's important to understand the nature of the immune system's response to viral infections and the specific characteristics of the H1N1 virus to address this question accurately.

**Step 1: Understanding the Immune System's Response to Influenza Viruses**
The human immune system is a complex network of cells, tissues, and organs that work together to defend the body against harmful pathogens, including viruses like the influenza virus. When the body encounters a virus for the first time, it mounts a multi-faceted immune response. This includes the production of antibodies, which are proteins that can specifically recognize and neutralize the virus. Additionally, the immune system activates T-cells, which can kill virus-infected cells and help regulate the overall immune response.

The Role of Antibodies and Immune Memory
One of the key aspects of the immune system's response to viruses is the development of immune memory. This means that after an infection, the body retains a population of memory cells that "remember" the virus. If the same virus is encountered again, these memory cells can rapidly initiate an immune response, often preventing the virus from causing disease or significantly reducing the severity and duration of the illness.

The Specificity of the Immune Response
The immune system's response is highly specific. It targets specific proteins on the surface of the virus, known as antigens. The H1N1 virus has unique antigens that distinguish it from other strains of the influenza virus. Therefore, if a person has been infected with and recovered from H1N1, their immune system should be better equipped to handle subsequent exposures to the same virus.

Step 2: The Uniqueness of the H1N1 Virus
The H1N1 virus is a type of influenza A virus. Influenza A viruses are further categorized by two types of proteins on their surface: hemagglutinin (H) and neuraminidase (N). The "H1" in H1N1 refers to the type 1 hemagglutinin protein, and "N1" refers to the type 1 neuraminidase protein. These proteins are the primary targets of the immune system's response.

Antigenic Drift and Shift
It's important to note that influenza viruses are known for their ability to change through two processes: antigenic drift and antigenic shift. Antigenic drift involves small, gradual changes in the virus's surface proteins over time, which can lead to new strains against which the immune system may have reduced effectiveness. Antigenic shift, on the other hand, is a more abrupt change that can result in a new subtype of the virus, to which the population would have little to no immunity.

**Step 3: The Likelihood of Reinfection with H1N1**
Given the immune system's ability to develop resistance to a specific virus and the specificity of the immune response, it is generally not likely for a person to be infected with the identical H1N1 virus more than once. However, due to antigenic drift, a person could potentially be infected with a slightly different strain of H1N1 or another influenza A virus. This is why annual flu vaccines are updated to match the most prevalent strains expected to circulate in the upcoming flu season.

Conclusion
In conclusion, while it is not common to contract the exact same strain of H1N1 influenza virus twice, the dynamic nature of the virus and the potential for new strains means that vigilance and preventive measures, such as vaccination, are important for maintaining health. The immune system's memory and the specificity of the response to the virus are the primary reasons why reinfection with the identical H1N1 virus is not likely.

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Getting infected with any influenza virus, including 2009 H1N1, should cause your body to develop immune resistance to that virus so it's not likely that a person would be infected with the identical influenza virus more than once.Feb 10, 2010read more >>