Firdaus Baskara, 8, of suburban Jakarta, Indonesia, survived bird flu. He's believed to have contracted the illness from his aunt.

Dr. Robert Webster, whose vaccine the U.S. government plans to use in case of an outbreak, told CNN at the time, "If this virus learns to transmit human to human and maintains that level of killing, we've got a global catastrophe."

That worldwide pandemic hasn't yet materialized, and bird flu has been out of the headlines for a while. But we may be in for another round of news.

Last week South Korea announced two new outbreaks in poultry. And Dr. Timothy Uyeki of the U.S. Centers for Disease Control and Prevention said he's bracing for another surge in human infections. "When the temperature drops and the humidity drops, that's when you start seeing more poultry outbreaks. And when you see poultry outbreaks, that's when you see human cases."

"It's still smoldering," said Dr. Anthony Fauci, who heads U.S. scientific efforts to combat bird flu. "What it hasn't done, much to our relief, is to become more virulent or better able to transmit from person to person."

New research

Three recent papers in the New England Journal of Medicine illustrate serious roadblocks to understanding and controlling the virus. The first describes three clusters of cases within families in Indonesia, eight patients in all. In two of the clusters, the authors said it's quite possible one person caught the disease, then passed it to family members.

One of those families was profiled in "Killer Flu," a CNN program last December. Rini Dina, a 37-year-old woman in a Jakarta suburb, died of an H5N1 infection, and her 8-year-old nephew, Firdaus, was hospitalized with fever for 10 days. Indonesian health officials said Rini was probably infected by tainted fertilizer in her garden. Firdaus had no direct contact with birds or the garden, but cuddled with his aunt on a couch while she lay shivering with fever. According to medical detectives, that's probably how he got sick.

"We can't prove it, but it probably happened," said Dr. Uyeki, who helped an Indonesian team with the investigations. Worldwide, about a third of all cases involve family clusters and there are a handful of cases where the virus likely passed from person to person, he said.

As a clinician, Uyeki has also helped to examine bird flu patients in Indonesia and Vietnam, the only U.S. doctor to do so. As the virus evolves, he said, its symptoms are evolving as well. "The clinical features in 1997 were different than what they are now. We're seeing less diarrhea, and in Indonesia, it's been much more fatal." Other, more common symptoms are hard to distinguish from other infections -- fever, aches and coughing, and shortness of breath and pneumonia as the illness progresses.

Simply making a diagnosis can be difficult. A second NEJM paper describes eight human patients in Turkey, all of whom initially tested negative for H5N1. The first samples were taken by swabbing the patients' nasal passages. That's standard for most influenza tests, but Uyeki says the H5N1 virus embeds itself deeper in the throat and lungs.

In Turkey, all the patients had been in contact with sick birds, so doctors were looking for H5N1. But in less-suspicious cases, a delay could be fatal. The only known treatment is the drug oseltamivir, sold as Tamiflu. While data are scarce because of the small number of human cases, most experts believe Tamiflu can lessen the symptoms of bird flu -- as it does with typical influenza - but only if taken in the first two days after symptoms appear.

In a commentary published with the two articles, Webster and another prominent flu expert said efforts to eradicate the virus, through killing infected chicken flocks or by vaccinating poultry, have largely failed. Worse, they said, many vaccines used in Asia are of poor quality and are pushing the virus to mutate faster, in potentially more dangerous directions.

So far, H5N1 hasn't gained the ability to easily infect humans, but eyebrows went up during an outbreak in May, on the Indonesian island of Sumatra. Bird flu infected eight people in a single family, killing seven of them. The World Health Organization concluded the illness had spread among the victims, but said genetic testing did not show major changes in the virus. Fauci said the H5N1 virus is indeed "a moving target," but that the existing vaccine provides at least some protection against the different strains. Work on more advanced vaccines is ongoing, he said. "Our capability is getting better and better, but it's not going to happen overnight."

"It's been a really rare human disease to date," Uyeki cautions, but "who can predict what's going to happen? We better continue to monitor and plan. To ignore this would be insane."

Caleb Hellerman is a producer with CNN Medical News.