Almost as quickly as it appeared, swine flu has faded as a public health threat - for now. But that could change by winter.
Public health experts say the virus has a chance of returning in a new and more dangerous form when the influenza season picks up again in late November, and are deciding now how to protect people worldwide.
They are preparing even as an increasing number of infectious disease experts are saying that the swine flu does not show signs of reappearing in a more virulent form next winter. It could even disappear altogether or more likely, come back and join the other strains of usually mild flu that typically circulate among humans each winter, they say.
But no one knows for sure.
"I wish I could predict what we are going to see, because it would be very easy to make the kinds of decisions we need to make as a nation over the next few months," said Dr. Richard Besser, acting director of the Center for Disease Control and Prevention, in a news conference late last week.
The CDC has started growing the virus in labs as a first step toward making a swine flu vaccine, and World Health Organization leaders will meet Wednesday to discuss when - and how - global manufacturing of a vaccine should start.
At the same time, infectious disease experts will spend the next few months studying the swine flu - a strain of influenza A, subtype H1N1 - to figure out just how dangerous it is, or could become. And public health experts will prepare for ways to keep the swine flu contained if there is another outbreak, including devising plans to close schools again when necessary.
Even if swine flu turns out to be no more virulent than seasonal flu, that's nothing to dismiss - seasonal flu kills about 36,000 people every year.
The flu is a respiratory illness caused by the influenza virus. Typically three strains of flu infect people every winter - two types of influenza A and one type of influenza B.
One of the distinguishing traits of influenza is its ability to mutate quickly and easily. The virus is made up of eight segments that will shift as the disease replicates. That leads to frequent small mutations, and when two different strains of influenza are in one person, or animal, the strains can mix and create significant mutations.
"That's why we have to vaccinate every year," said David Lewis, an infectious disease specialist with Stanford University School of Medicine. "There are always changes."
"Every time influenza replicates itself, it has a tendency to naturally make some mistakes," he said. "That means it's generating a whole bunch of slight changes that can be selected for a slight advantage. So it's a very rapid natural selection for whatever virus is able to elude the immune system."
That's what scientists believe happened with swine flu, which is a mix of viruses found in pigs, birds and humans. What alarmed infectious disease experts was the virus' ability to pass from person to person - a trait that could prove devastating if a virus is particularly harmful to humans.
So far the symptoms of swine flu appear to be mild, especially in the cases found in the United States. More than 1,600 confirmed cases of swine flu were reported in the United States over the past three weeks, about 50 people were hospitalized and two died. On Saturday, health officials in Washington state reported a death that also may have been caused by swine flu.
In the early days of the outbreak in Mexico, it seemed to be a serious health threat because it killed more than 150 people. But epidemiologists believe many thousands of mild cases of swine flu in Mexico were never reported - making it no more deadly than seasonal flu.
Why experts worry
But this particular strain is new to humans - leaving people with little natural immunity to it - which would mean more people would be vulnerable to infection. Given that vulnerability, a more virulent form in the winter would be all that much more worrisome.
So public health experts are preparing. The most famous influenza epidemic in recent history started in the spring of 1917 with a mild outbreak of the flu that returned months later and killed millions worldwide.
"I don't think this (swine flu) is going to be a pandemic. I worry about a true pandemic with a wholly new strain (of influenza) more than I worry about this one. But you can't ignore it," said Dr. Larry Drew, director of the UCSF virology lab. "We're going to have to address this strain."
The most important step, say infectious disease experts, will be preparing a vaccine. But public health officials have difficult decisions to make right now if they intend to have a vaccine ready by next winter.
They must decide whether the swine flu vaccine should be combined with the seasonal flu vaccine, which is already designed to combat three forms of influenza. Some vaccine experts worry that adding swine flu into the mix will dilute the protective abilities of the rest of the vaccine - and argue there probably won't be enough time to test the efficacy of combining all four.
If swine flu can't be combined in the usual vaccine, a separate one will have to be made. But that raises questions of how many vaccine doses to prepare worldwide, and whether developing a new vaccine will interrupt manufacturing of the seasonal vaccine.
Who gets vaccinated
Then there's the question of who would be best served by a vaccine. With swine flu getting so much attention, demand will almost definitely be higher than normal this year, say public health experts.
"Given the lead time, we probably have the ability to make a vaccine, at least for the United States. But part of it depends on how many people you're planning on vaccinating," Lewis said. "This next season there will probably be a real crush to get vaccines."
If there aren't enough vaccines to give to everyone who wants them, priority will go to the most vulnerable - young children and the elderly, along with people who have other health risks or who work in health care.
Since early evidence suggests that young people - particularly teenagers and young adults - make up most of the cases of swine flu, it may be logical to target a vaccine to high school and college students, said Drew. That's yet another piece of the swine flu mystery that researchers will be studying this summer.
Meanwhile, Besser of the CDC is urging local public health officials and communities to be prepared for the worst.
"What would they do if this came back and was really severe and we needed to take measures like closing schools?" he said. "That kind of preparedness over this period of time will really help minimize the impact, should this come back and be more severe."
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