Government officials, public health experts and business leaders have faced a lot of questions about swine flu this week, and three words keep popping up in their answers: "We don't know."
The flu outbreak has spawned many mysteries _ about origin, scope and impact, just for starters _ and some of the most fundamental questions remain unanswered, or unanswerable.
Here's some of what we don't know.
1. How many people have had the virus?
A lot of numbers have bubbled up _ more than 100 lab-confirmed cases in the United States, more than 300 in Mexico, and dozens more in a few other countries. And then there are the suspected cases _ thousands in Mexico alone.
But the real number of infections around the world is impossible to know, since many people may contract swine flu and simply get better without ever seeing a doctor. Even among the lab-confirmed cases of swine flu, many people have had only mild symptoms, similar to a run-of-the-mill case of seasonal flu.
2. How big will this get?
This is the million-dollar question, and we probably won't know the answer until scientists figure out how easily the virus spreads and how often it kills. Then, they might be able to project more accurately what damage the outbreak might cause.
An influenza outbreak can spread rapidly and unpredictably, requiring only a cough or sneeze to jump from one person to another. The swine flu has taken hold in densely populated areas _ like Mexico City, home to 20 million people _ where it can spread at an alarming rate. And the disease has popped up in many parts of the world, thanks to easy travel between one continent and another.
Even the public health experts most plugged into swine flu don't want to speculate about the ultimate scope of the outbreak.
There have been some clues about the outbreak's future, though: On Thursday, Mexico's top health official said the number of new swine flu cases is stabilizing, at least in that country. Though WHO's flu chief, reacting to similar comments from other Mexican officials, said WHO hadn't seen evidence of leveling off and cautioned that case numbers often go up and down, particularly in the early stages of an outbreak.
3. When and where did it start?
The virus may have mutated into its current form months or even a year ago, and it happened, well, somewhere in the world.
The first symptoms started to show up in early March in the Mexican state of Veracruz, an area with a number of pig farms. The earliest confirmed case was a 5-year-old boy, one of hundreds of people in the town of La Gloria whose flu symptoms left them struggling to breathe.
People from La Gloria kept going to jobs in Mexico City despite their illnesses, and could have infected people there.
Still, there's no guarantee the virus came into existence in Veracruz. It spreads so easily, it could have made its way to Mexico from just about anywhere.
Some Mexican officials have offered some other suggestions of places where the virus may have begun, such as China, Pakistan or Bangladesh. But the fact is that the flu's origin remains a mystery.
Fortunately, it also doesn't really matter _ the outbreak has made its way around the world, and nothing can be done now at the point of origin to slow it down.
4. Why have so many people died in Mexico, but virtually nobody anywhere else?
OK, this is the REAL million-dollar question, and a swarm of public health experts _ including several from the U.S. Centers for Disease Control and Prevention _ have descended upon Mexico in search of an answer. This is especially a head-scratcher because experts have found no difference in samples of the virus collected in Mexico and the United States.
One theory is that many more people have had the virus in Mexico than health officials realize, and the number of cases there could well eclipse the number anywhere else in the world. If so, that country's death toll _ more than a dozen confirmed deaths and many more suspected ones _ would make sense.
Other theories have been floated _ and ruled out _ for the number of deaths in Mexico:
_ Lab tests of Mexican patients found no sign of complications from a second infection.
_ CDC investigators have not seen any obvious problem with low-quality health care.
_ The CDC found no evidence of an over-the-counter medicine or folk remedy compounding the problem.
_ Complications from Mexico City's altitude or air pollution are unlikely because severe cases have been reported in parts of Mexico at low altitude and with cleaner air.
5. Who are the victims in Mexico?
The Mexican government has revealed little about the victims, citing privacy reasons. The government has even been a bit haphazard about providing information that would not violate privacy rules, such as ages and hometowns.
After prodding by journalists, Health Secretary Jose Angel Cordova revealed Thursday that 5 of the 12 confirmed dead by that point were between the ages of 20 and 40 and that they had an overactive immune system _ possibly explaining why they did not survive.
On Wednesday, when there were 99 confirmed cases, he said 83 of them were from Mexico City. But when the number jumped Thursday, no information about hometowns was offered.
Also, Mexico's chief epidemiologist, Miguel Angel Lezana, told reporters Wednesday that one of those confirmed dead was a Bangladeshi who had been in Mexico for six months, and whose brother _ also apparently sick _ had recently visited from Bangladesh or Pakistan.
And on Thursday, Mexico City government officials announced that preliminary investigations showed most of the people suspected to have died of swine flu in the capital lived in poor neighborhoods.
6. Will changes in the WHO alert level help stem the outbreak?
That's the idea, but it's unclear what concrete actions are actually taken by WHO and countries with a change in the alert level _ which was bumped up on Wednesday to phase 5, one step away from the highest level, which indicates a global outbreak.
The alert levels mainly signify WHO's assessment of the pandemic situation, but they do come with actions and responsibilities. At the higher alert levels, WHO is essentially warning countries to prepare for a pandemic.
WHO monitors the outbreak situation at every level, but surveillance increases at higher levels for unusual outbreaks, the disease's spread, and the virus' possible drug resistance. WHO may also issue guidance about travel advisories, border closures, closings of schools and offices, and suspension of mass gatherings such as sporting events.
The alert system, however, is largely untested. Monday was the first time it had ever been raised above phase 3, which signifies only occasional cases or small clusters of a new flu virus.
Also, it's ultimately up to individual governments to activate pandemic response plans and to take such steps as closing schools or workplaces where a disease might spread. WHO cannot force countries to comply with recommendations.
At phases 5 and 6, WHO will also consider asking vaccine producers to switch from making seasonal flu vaccine to pandemic vaccine. It will also oversee distribution of its emergency stockpile of 5 million antiviral treatments to countries in need and help negotiate with vaccine makers for a proportion of the vaccine to go to developing countries.
How much does all this help? Until an outbreak has come and gone, the truth is: We don't know.
Associated Press writers Mike Stobbe in Atlanta and Alexandra Olson, Mark Stevenson and Andrew O. Selsky in Mexico City contributed to this report.