Turns out I was mistaken. See below the fold...
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Mexico City is a huge urban area with extensive suburbs. In those suburbs, which span the range from well-off to desperately poor, people commonly keep pigs and chickens in their yards, as part of their household food supply.
The pigs are fed on household food wastes: table scraps, garden wastes, and so on, that are not cooked (sanitized) before being fed to the pigs. When chickens are slaughtered, the uncooked chicken entrails are also fed to the pigs.
This is the same situation as occurs in the rural areas of China, except that in China people keep ducks along with their pigs at home. In China the usual route of virus propagation is from wild foul to domesticated ducks, then to pigs, then to humans.
This is what most likely occurred in Mexico. More below, along with a prediction or two...
The pig/poultry mechanism is well-understood by public health experts so it appeared to be a logical candidate for the origin of H1N1. But not this time; apparently the virus originated in a North Carolina hog farm, one of the largest in the US. Go here:
http://www.dailykos.com/...
So my speculation was mistaken, and that's that. (Update as of 2009 May 05)
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This is what scientists will probably find to have happened: (No, they didn't. See paragraphs above.)
(Note: all of this is purely my own speculation, based on my knowledge of public health. However, folks here who have relevant science background will probably find my reasoning is sound. Or if not, please feel free to tear this to shreds and suggest other hypotheses.)
A new avian virus originated in the wild. The migration of wild birds carried the virus into Mexico City and other populated areas. Typically this occurs as wild birds fly over an area and their droppings fall into poultry pens.
The wild bird virus got into the household chickens. It was highly infectious, which means highly transmissible, so it spread quickly though outdoor markets and other places where people come into contact with chickens other than the ones they themselves are raising at home. However it was not highly virulent, which means it was not causing an increase in the death of chickens. Thus, unlike Avian Flu H5N1, it was not immediately noticed in the chicken populations.
From the chickens, the virus passed to the household pigs. This would have occurred in two ways. One, from pigs and chickens being kept together in the yard, where the pigs came into contact with chicken droppings. Two, from the pigs being fed chicken entrails along with other uncooked household food wastes. Thus the flu virus in the chickens combined with other flu viruses normally found in swine. This is how we got the combination of avian and swine flu genes.
Human flu genes got into the virus in two ways. One, humans with normal seasonal flu fed their dinner table food wastes to their household pigs. Two, humans with seasonal flu interacted with their household pigs in the yard. Thus the human flu genes mixed with the avian & swine flu genes present in the pigs.
The new combined virus was transmissible from pigs back to humans, and was also H2H (human-to-human) transmissible. Thus it passed among humans. As often happens with new viruses, the virus was most virulent (caused the worst symptoms and highest death rate) among those who were in closest proximity to the source of initial infection. This would account for the high CFR (case fatality rate) among the cases in Mexico.
So now you have the "ground zero" infections: people who caught it from pigs, or from people who had high contact with pigs. These people got the worst of it and had the highest death rate.
After that came the "first ring" infections: people who caught it from the "ground zero" cases. CFR would be starting to decrease in this population.
After that, the "second ring" infections: people who caught it from the first ring cases. CFR would decrease further in this population.
After that, the "third ring" infections: people who caught it from the second ring cases. This would most likely be the people in the US who caught it. Notice that so far, the CFR in the US is zero: no deaths. This would be consistent with the virus losing some of its virulence as it passed between humans: a common occurrence for new viruses.
And this is what's probably going to happen next:
As the virus spreads, it will lose virulence. Cases will continue to occur in the US and eventually the rest of the world. However, the CFR will decline until it is approximately the same as for normal seasonal flu.
Thus we will have ducked a bullet. This time. Next time we may not be so lucky. Avian flu H5N1, still remains to go H2H transmissible, and when it does, it'll be a doozy.
The moral of the story is: sanitation is still the first line of defense against infectious disease. The unglamorous measures of washing hands, properly preparing food, and properly disposing of wastes, are still the most important measures for public health.
--- update #1 ---
NOTE: this is NOT to suggest complacency (UPDATE):
Some comments below indicate that I may be suggesting we can relax and not worry about this virus. That's not what I'm saying. This virus could after all turn out to be a Big One. And it may still turn out to cause fatalities in more distant "rings" as it continues to spread. To be quite clear, I do think this will go pandemic in the technical sense of spreading worldwide. And we still have got to deal with it as if it is a Big One, because we should never take chances with new viruses.
So, y'all should still be taking preparedness steps, and following preventive measures, as if this is a Big One. At very least, it's a good practice drill. And if this turns out to be worse, you'll be on safer ground.
--- end of update #1 ---
And the political aspect is:
The present health system in the US, based on private insurance and with 20 - 30% of Americans not covered, is a setup for disaster. Viruses do not care about the socioeconomic status, immigration status, or party affiliation of the people they infect.
The CFR of Spanish Flu was about 6%. The CFR of the first and second rings of the current swine flu, has been about 8% (and rapidly came down from there on its own). The CFR of avian flu H5N1 in humans at present is 65 percent. Containing a truly dangerous flu outbreak and preventing a slate-wiper pandemic, will absolutely require a public health system that covers 100% of the population.
The only way to get there is with a universal single-payer system.
This is the lesson we need to take from here to Congress and the White House: this time we may have ducked a bullet, next time we may not be so lucky, and universal single-payer health care is the way to prevent a slate-wiper.
--- Update # 2 below ---
From the comments: Mariachi Mama: "...over the past several months, there has been a severe water shortage in some of these areas. There have been numerous protests about the lack of water in some of these sprawling suburbs."
Water shortage --> people can't hose down the areas where pigs & chickens are kept --> increased accumulations of droppings --> increased transmission of viruses.
This is a perfect example of the connection between the climate crisis and public health issues.
Meanwhile, what you should be doing about this:
Wash hands after using the toilet, before preparing food, before eating, and after getting home from work or school, and after handling money (yes, money is a vector path, because it passes through so many hands). Don't put unwashed hands to your face: no rubbing of eyes, picking of noses, scratching of ears, or fingers in mouths.
Maintain separation between meat and vegetable preparing utensils (and pots, pans, plates, etc.), and between those used for raw meat and for cooked meat.
Keep food wastes in closed containers before collection or composting.
AVOID crowded places: movie theatres, concerts, clubs, malls, churches, etc.
Do your grocery shopping at times when the store is less crowded: mornings and nights, not evening rush hour. While in a grocery line, leave a shopping-cart distance between yourself and the person ahead of you; and pull your shopping cart in AFTER you to keep the person behind you at that distance in case they are trying to crowd the line. Shop further ahead if possible: buy more groceries on each trip so you have to make fewer trips.
Don't share food or utensils from person to person: no sharing a bottle of soda, no eating off the same plate, no using your utensils to take food from a common serving dish, and (since it's Saturday night, let's be pragmatic here) no passing the bong around at parties.
If you live in an area where the air is particularly dry: humidify the air in some way; higher humidity decreases virus transmission through the air. You can use a humidifier, or you can hang wet towels in living spaces. (Use an indoor drying rack for your laundry: this will also save quite a bit on your electric & gas bill compared to using the clothes dryer.)
Get your sleep: gives your immune system a nightly boost. Drink plenty of water: helps your body flush viruses out faster, and provides for an adequate supply of nasal mucus to trap viruses before they get to you.
--- end of update # 2 ---
--- update # 3 ---
A few words about the history of pigs & garbage.
In public health usage, "garbage" refers to food wastes, and "rubbish" refers to dry non-food wastes (most of which we now consider recyclables).
Pigs eat just about anything, and this fact has historically been put to good use by feeding them garbage. By the early 20th century, when cities were starting to provide garbage collection service, the municipal garbage would typically be sold to hog farmers: they got cheap hog food, municipalities got funds to help offset the costs of collection.
European health authorities quickly discovered, by about the 1930s, that feeding uncooked (un-sanitized) garbage to hogs, was a major public health risk. For one thing, it appeared to amplify trichinosis infections. Thus they required that garbage be sanitized by cooking, before feeding it to hogs. This was typically done with steam injection or in bulk kettles.
Americans were behind the ball on this one, so American pork was forbidden from importation into Europe at the time. By the 1950s, American practices had caught up with European practices, and garbage had to be sanitized by cooking before it could be fed to hogs.
In the US, where garbage was collected for hog feeding, it was typically handled separately from rubbish. Thus you would have two refuse bins: a smaller one for garbage and garden waste, a larger one for rubbish. These would be picked up by separate crews on separate trucks, for example garbage on Tuesday & Friday, and rubbish on Friday only. The garbage would be taken to the hog farm, the rubbish would be taken to an incinerator or open dump (later, to a sanitary landfill where it would be buried in trenches and covered with earth every day). In some places, ashes from household coal-fired heating systems were also collected separately, and used as a sterile covering material in landfills.
By the 1960s, the landfill system was in common usage, and separate collection of garbage was abandoned in most places by 1970s: all household wastes were landfilled. By the early 1980s, landfill space began to run out in most areas, and this is ultimately what led to the nearly universal implementation of source-separation recycling. Today, recycling plants are being built that enable mixed municipal refuse to be sorted to reclaim recyclables. Thus we are seeing a return to mixed collections in many areas, or at minimum, to all recyclables being collected together (paper, glass, plastics, metals, in the same bin).
Of course no matter what else, pork still has to be cooked to well-done (white, not pink) before eating.
And no matter what else, keeping pigs & chickens together, is still a major route to the development of novel viruses that can pass to humans and cause pandemics.
--- end of update # 3 ---