Holly told me that my last post was too dry and that I should make it more personal, so here goes:
Influenza fascinates me. Next to malaria, I’ve spent more time reading about flu than any other disease. Like malaria parasites, the surface antigens on influenza viruses change and allow flu viruses to hide from the immune system. This will be important when we talk about flu vaccine efficacy and effectiveness.
I became interested in flu in 2005 when I was involved with avian influenza pandemic preparation at the hospital where I worked at the time. My first direct experience with pandemic influenza was in 2009 when a pregnant woman was admitted to the intensive care unit (ICU) with what was then called “swine flu” by the media. Later that year, I worked for the Louisiana Office of Public Health in 2009 H1N1 pandemic influenza response.
There are three types of flu viruses: influenza A, influenza B, and influenza C.
Influenza C causes mild disease in humans, and that's pretty much all you will ever hear about it. We don't bother vaccinating against it.
Both influenza A and influenza B cause severe disease in humans. Influenza B only infects humans. Influenza A infects humans, other mammals, and birds. In fact, birds are the reservoir for vast number of influenza A viruses.
Viruses are not cells. They do not have the cellular mechanisms required to manufacture proteins. They cannot replicate (make copies of themselves) independently. Viruses infect living cells and take over their "machinery" to replicate. Not surprisingly, influenza viruses kill the cells that they infect.
Influenza viruses have special glycoproteins (large molecules that are made up of sugar and protein) on their surface. Hemagglutinin, the "H" of H1N1, H3N2, and H5N1, allows influenza viruses to attach to and penetrate cells in the respiratory tract. Neuraminidase, the "N" in the virus name, allows newly formed flu viruses to escape from the infected cells.
For influenza A viruses, there are at least 16 different hemagglutinins and 9 different neuraminidases, so there are influenza A permutations from H1N1 to H16N9. Humans are most commonly infected with H1, H2, and H3 and N1 and N2.
The ability of a flu virus to infect humans depends on how well it is adapted to receptors in our respiratory tract. Influenza viruses that are easily transmitted from person-to-person are those that are best adapted to human receptors. Humans can be infected with influenza A viruses that are adapted to other mammals or birds, but usually only through close contact with infected animals.
Hemagglutinin and neuraminidase are two of the antigens targeted by our immune system. When we are infected with a flu virus, our immune system starts producing antibodies that are specific to the hemagglutinin and neuraminidase on that virus.
The genes that code for hemagglutinin and neuraminidase mutate frequently, creating minor changes in those antigens. When enough of these changes accumulate, our antibodies no longer "recognize" that virus. This is called antigenic drift.
Both influenza A and influenza B drift, but only influenza A viruses shift.
That is a subject for another time.
Hayden, F. G. (2011). Influenza. In L. Goldman & A. I. Schafer (Eds.) Goldman's Cecil medicine (24th Ed.) [Electronic version]
Treanor, J. J. (2009). Influenza viruses, including avian influenza and swine flu. In G. L. Mandell, J. E. Bennett, & R. Dolin (Eds.). Mandell, Douglas, and Bennett’s principles and practice of infectious diseases. (7th Ed.) [Electronic version].