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An international team of disease detectives are in China to investigate an outbreak of a new strain of bird flu, H7N9. The biggest puzzle right now is where these infections are coming from, as testing poultry has turned up very few infected birds.

A new strain of bird flu has sickened 82 people and killed 18 in China. But many people who have caught the H7N9 flu say they hadn't been near poultry or other birds. So what's fueling the outbreak of the virus?

Health officials in China say they've confirmed 11 new bird flu diagnoses, bringing the number of H7N9 infections to 60, according to the official Xinhua news agency. The virus, which began in eastern China, has now sickened at least one person in Beijing, and two in the central province of Henan.

"The top priority is diagnosis — the capability to be able to pick up this virus, should it emerge outside of China," says virologist John McCauley. Flu researchers are getting started on creating a vaccine, but there are still many unknowns.

Infections with a new flu strain have increased, with three to five cases reported daily. The virus, carried by birds, doesn't appear to spread between people. Still, health officials in the U.S. are preparing to screen travelers and develop a vaccine.

Scientists are scrambling to understand a bird flu virus never before found in humans. It grabbed world attention this past week after it infected and killed people in China. Weekend Edition Sunday host Rachel Martin talks with Dr. Thomas R. Frieden, director of the U.S. Centers for Disease Control and Prevention.

Sixteen cases of a new flu in China have touched off a major effort to determine what kind of threat it might be. Flu experts want to know where the H7N9 virus is coming from and how it gets around.

Why do people in Boston get the flu when it's cold, while people in Senegal get sick when it's hot? Humidity is a big part of the explanation. But how flu spreads in the tropics and more temperate climates appears to be different.

This year's flu shot looks like it's unusually poor at protecting the elderly. The flu vaccine's only about 27 percent effective overall for those ages 65 and older and just 9 percent effective against the flu strain causing the most illness, according to the federal Centers for Disease Control and Prevention. Melissa Block talks to Rob Stein.

It appears that the current batch of flu vaccine is only about 9 percent effective in protecting people 65 and older against the H3N2 flu strain that's causing the most illness. Even so, health officials still recommend vaccination.