"We've got many, many months ahead of us where we don't know what will happen and we need to take the best steps we can to protect ourselves," Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, said last week. "Our biggest concern is that the virus could change, mutate to become more deadly."
With winter approaching, another fear is a one-two punch in which a resurgent swine flu batters young people before the vaccine is widely available, while the ordinary flu strikes the elderly. Also, emergency doctors are worried about the strain on ERs and hospitals.
To date, swine flu has hospitalized hundreds of thousands of people around the world and killed at least 4,500, including at least 600 in the United States. At least 81 U.S. children have died, including many who had no underlying health problems.
The CDC was the first to identify the new flu. It was on April 15 that the agency determined the nasal samples from two children in Southern California revealed a swine flu virus that had never been seen before. It was found to contain bits of bird and human flu.
At first the cases represented more of a scientific puzzle than a public health threat. The two children recovered, but investigators were perplexed by how they got it, since the two kids had not been in contact with each other or with pigs.
But within a week, the situation became more dramatic, when testing linked the two children and a handful of subsequent U.S. cases to hundreds of illnesses in Mexico City. Mexican authorities closed schools, museums, libraries and theaters to stop the spread of the disease as initial reports suggested it was killing as many as one in 15 of those infected - a horrifying death rate more than three times higher than the terrible flu pandemic of 1918-19.
A series of bad and good news followed. First, the bad: It quickly became clear that the virus was spreading not only in Mexico and border regions of the United States, but around the world. As health officials had long surmised, international air travel provided a rapid path to world contagion.
What's more, studies indicated the millions of seasonal flu shots administered the previous winter offered no protection against the unusual new virus.
But then came some good news:
-While the flu vaccine was no help, the antiviral medication Tamiflu reduced the severity of illness if taken right after symptoms appeared.
-People 55 and older, who suffer and die the most from seasonal flu each year, seemed mostly to be spared by the new virus. Scientists credited some immunity that they had perhaps picked up from exposure decades ago to a similar-enough virus or vaccine.
-Additional investigation in Mexico suggested that many people had suffered only mild illness. Those cases were not counted in initial reports, meaning the death rate was much, much lower than originally estimated.
In the United States, some of the initial response plans for the new swine flu, an H1N1 strain, envisioned "people dropping dead in the streets," recalled Dr. Beth Bell, a CDC epidemiologist who has been a leader in the agency's response.
The disease kept spreading, and eventually the World Health Organization declared it the first global flu pandemic in 40 years. But even before then, U.S. health officials had downgraded some of their prevention advice, such as the call for schools to shut down for two weeks if any students became infected.
"Overall, it's a fairly typical flu virus," Richard Webby, a prominent researcher at St. Jude Children's Research Hospital in Memphis, says now.
But this story is not over yet. There are still important unanswered questions. Most health experts believe swine flu hits children and pregnant women harder than seasonal flu, but it's not clear how much harder because officials don't know exactly how many have caught the swine flu and had only mild symptoms.
A mutation of the virus seen in two Dutch patients last month at first seemed to indicate the bug might be getting more dangerous, but the patients recovered and no further problems were reported. Researchers are watching for more such changes.
Over the summer, new infections spread, often hitting kids' summer camps. That's unusual because seasonal flu usually disappears in summer. But it wasn't a shock, either, because summer spread had been seen in flu pandemics of the past, like one in 1957, said Dr. Arnold Monto, a University of Michigan flu expert.
Experts predicted, correctly, that infections would jump in August and September, when schools and universities opened for fall classes.
Now, manufacturers are cranking out vaccine as fast as they can around the world, and the early shipments are trickling out.
Some health officials have speculated the worst may already be over for some parts of the country, particularly the Northeast. If the fall vaccination campaigns are effective, swine flu's winter season may not be as bad as the spring.
However, some experts think things could get worse. Some believe swine flu will be the dominant virus, as it has been during the Southern Hemisphere's winter, just ending. Others worry about the double-flu scenario in which the novel H1N1 virus strikes the young and the ordinary flu socks the old.
In either case, hospitals could be swamped. That's been causing a lot of hand-wringing, with a poll of 1,000 emergency room doctors this week finding that 90 percent are worried about their hospitals' ability to handle extra patients.
Whatever happens, health officials say it's been a memorable experience.
"There is a certain sense of history unfolding," Bell said.
On the Net:
World Health Organization swine flu site:
U.S. Centers for Disease Control and Prevention swine flu site: http://www.cdc.gov/H1N1FLU/