Swine flu, bird flu, Spanish flu, Asian flu, and most other named flu epidemics are caused by the influenza A virus. Both influenza A and influenza B cause seasonal epidemics, but the A type has additional potential to cause global pandemics.
Dr. Conall McCaughey of Royal Hospitals, Belfast, UK, provided a review. The names H1N1, H3N2, etc. refer to subtypes characterized by their surface proteins hemagglutinin (H) and neuraminidase (N).
McCaughey listed eight recorded major outbreaks caused by influenza A:
1. In 1889, H2N2 pandemic, estimated deaths of 1 million.
2. In 1900, H3N2 pandemic, estimated deaths of < 1 million.
3. In 1918, H1N1 “Spanish” flu pandemic, estimated deaths 20 to 100 million.
4. In 1957, H2N2 “Asian” flu pandemic, estimated deaths 1 to 1.5 million.
5. In 1968, H3N2 “Hong Kong” flu pandemic, estimated deaths 0.75 to 1 million.
6. In 1977, H1N1 “Russian” flu, not a true pandemic, estimated deaths < 100,000.
7. In 2002, H1N2, noted for reassortment between H1N1 and H3N2 viruses.
8. In 2009, H1N1 “Swine” flu pandemic, estimated deaths < 100,000.
“Two particular features of the influenza virus contribute to its rapid evolutionary capacity to change its antigens and escape immune recognition,” McCaughey explained.
The first is a high rate of errors during replication of the virus genetic material, causing rapid mutation. The second is the separation of the RNA into eight separate segments.
When two different viruses infect the same cell, segments can be swapped. This is equivalent to “sexual reproduction in viruses”.
“Most influenza genetic diversity is in birds,” McCaughey added. Influenza A infects wild birds, domestic birds, and swine, as well as humans.
Most seasonal influenza epidemics are caused by virus strains similar to those of previous years, so we have some immunity. When a new virus jumps from an animal host to humans, we are more susceptible to widespread infection.
Tasleem Samji, a graduate student at Yale University School of Medicine, provided a review of the life cycle of influenza A virus.