Bird Flu is coming
This is scary. Here is the basic scoop on the bird flu:
The "normal" flu virus varies a bit from year to year. That is why you need to get revaccinated year. The two main genes that vary are called hemagglutinin (H) and neuraminidase (N) which, not surprisingly, are exposed on the outside of the virus and therefore the immune system. You may see flu strains referred to as H1N5 or whatever to denote the strain based on hemagglutinin and neuraminidase sub-types.
If you can make antibodies that bind to these exposed portions of the virus they can block infection. To make these antibodies you either need to be vaccinated or previously exposed. Hemagglutinin and neuraminidase are under enormous selection pressure, so they mutate much faster than proteins on the inside of the virus, a nice example of natural selection at work,
Generally, the changes from year to year are small, so there is some level of residual protection at a population level.
Flu viruses also circulate among poultry and swine, but these viruses either can't infect humans or do so ineffectively. Because they are genetically different from the usual human viruses there is no history of previous exposure and so they are usually more deadly. But because they don't infect humans very well, they generally don't spread from person to person.
The big fear is that a person (or, interestingly, a pig) will be simultaneously infected with both the avian and the more typical flu viruses. This will allow the genetic materials of the two viruses to combine. This has the potential to generate a new virus has the deadly combination of ability to infect humans, but new antigenic determinants (H and N) on the outside which humans haven't encountered before and so aren't resistant against.
This leads to what is called a pandemic, where the new virus sweeps the world, killing millions.
The 1918 pandemic killed an estimated 20-40 million worldwide, more than WWI or the plague. Interestingly, mortality was highest among healthy young adults, unlike more typical years, when most death is among young children and the elderly. The reason for this, and whether it would be repeated in another pandemic is unknown (at least to me).
Preparations for a new pandemic are inadequate but people are starting to realize the need for them. The two most important things are to have the capacity to rapidly produce a vaccine in large quantities and to stockpile anti-viral drugs.
This could be really, really bad.