It was four years ago this month that a new strain of flu virus was reported in Mexico and captured global attention. Ultimately, the World Health Organization declared it a pandemic. More than 18,000 people died in 2009 from the virus.
Now, in China, global influenza experts are watching another novel virus, H7N9. So far, nine people are sick, and three are dead. As Helen Branswell at The Canadian Press (and self-described "flu freak") reports "those first three sick people and the genetic sequences of the flu viruses that infected them were enough to make the hairs on the backs of knowledgeable necks stand on end."
Branswell also tweeted that case "numbers are a moving target" right now.
She lays out why flu experts are racing to determine if we're on the brink of the second pandemic in five years in this report:
Influenza scientists always pay attention when animal flu viruses start making people sick. There is a rich soup of flu viruses in nature, most of which human immune systems have never seen. Many of those viruses, at least in theory, have the potential to trigger pandemics.
So when China notified the World Health Organization over the weekend that it had found three cases of infection with H7N9 viruses, there was immediate concern.
While some H7 viruses have infected people in the past -- including two poultry workers in a big H7N3 outbreak in British Columbia in 2004 -- viruses bearing an H7 hemagglutinin and an N9 neuraminidase in combination had never been previously known to infect humans.
Adding to the alarm: The three people were not linked. That means each caught the virus from an animal, or a person. The first bet would be animals, but so far the virus hasn't been found in Chinese poultry or pigs, the likely suspects.
And the cases didn't live close to one another. Two were in Shanghai, China's largest megacity. But the third was in the nearby province of Anhui. As more cases have emerged, the geographic distance among infections has become greater, making it increasingly clear that there is H7N9 virus spreading, undetected, over hundreds of kilometres in China.
Dr. Keiji Fukuda, the World Health Organization's top flu expert explains: "It's possible that we have infected animals. But we still haven't detected actually what is the infected animal and whether it's these animals that are spread out and individuals in a widespread area are coming into contact (with them) and becoming ill."
The other option? "At the same time you have to be thinking about is there any evidence of person-to-person transmission," Fukuda, the WHO's assistant director-general for health security and environment, told The Canadian Press in an interview.
When people show up in hospitals sick with pneumonia, it can take a while to figure out what bacteria or virus is making them ill. It takes even longer when tests for known culprits come back negative, and laboratories have to start thinking about whether they are dealing with something new.
The first known H7N9 cases started getting sick in late February. And two died in the first part of March. But once the Chinese hospitals got the country's public health agency -- the China CDC -- involved and they found the new virus, that organization quickly mapped the genetic sequences of the first three viruses and uploaded them into a database that international flu scientists can access.
That prompt sharing allowed outside experts to start studying the viruses to look for clues to their origin.
All eight genes of each virus were avian, meaning these were bird flu viruses. But all the viruses showed several genetic changes that scientists who study influenza recognize as signals of a virus adapting to spread in mammals, not birds. Those changes mean the virus is learning to attach to the type of cells people have in their upper airways, the type of cells that human flu viruses infect.
Those signs of adaptation ratcheted up the concern.
"We would be paying very close attention to any situation in which you have a novel influenza virus which has caused some cases of infection and caused some deaths," Fukuda says.
"But I think that the molecular changes also make us pay a lot of attention."
Richard Webby, a swine flu expert who runs the WHO's influenza collaborating centre at St. Jude Children's Research Hospital in Memphis, Tenn., says those genetic changes suggest the virus may no longer be circulating in birds.
He believes it is in mammals. That could mean pigs, which are called mixing bowls of influenza, because they can be infected with both human and bird flu viruses, providing a chance for genes to swap.
Or it could mean the virus is spreading in people.
Typically with influenza, we see a (relatively) small number of deaths, many very sick people and then a large group with such mild symptoms they might not even know they have the flu.
Right now, experts don't know if that's the case with this flu or if it's something much more serious.
Longtime pandemic-watcher and Pulitzer Prize winner Laurie Garrett filed this piece earlier this week for Foreign Policy where she examines whether the recent (and mysterious) mass deaths of pigs and ducks are linked to the flu deaths in people. "Is this a pandemic being born?" the headline asks.