Q. I heard that a dangerous new kind of "bird flu" was recently discovered in China. Should we in the United States be worried?
A. You're right: In March 2013, cases of a brand-new kind of bird flu were discovered in China. One of the hardest things to predict is what will happen when a new strain of the influenza ("flu") virus first infects humans. I'm not exaggerating when I say it could turn out to pose no threat at all in the U.S., or it could be truly terrible.
The world is full of different flu viruses. Most of them infect just animals, often just one type of animal. Sometimes they are able to jump from one type of animal to another. Sometimes they are even able to jump from animals to humans.
Flu viruses are so unpredictable because they are constantly changing or mutating their genes. (Genes are sections of DNA that control specific characteristics of an agent, such as a flu virus.) They also are constantly swapping their genes with the genes of other flu viruses. When they change or swap their genes, they change their character.
There are two questions to ask about the character of every new flu virus that affects humans: how easily is it spread from one person to another, and how sick does it make the people it infects? Its assortment of genes determines the answer to those questions.
When a new virus spreads easily between people, and if it makes people very sick, there is a lot to worry about.
How bad can a flu virus be? The worst flu outbreak for which we have good records occurred in 1918. A new virus was born that spread easily between people and made people very sick. In the course of a few months, 30 percent of the people in the world caught the virus, and at least 20 million people died. That's more than were killed in World War I.
From what we know about this new flu virus, called H7N9, it has jumped from chickens and ducks to humans in eastern China. It is not yet clear whether it has spread from one human to another. It has made some people very sick and there have been several deaths.
The latest numbers were reported by the U.S. Centers for Disease Control and Prevention (CDC) on April 29, 2013. China had reported 126 human cases of the new flu. The people affected have been older (average age in their 60s), and many have had chronic illnesses. So far, many fewer children and young adults have been infected.
The CDC and scientists at the National Institutes of Health (NIH) are working overtime to protect our health. They are doing what they can despite the fact that the funding for these two critically important federal agencies has been cut by the "sequester." Regardless of what you think about federal spending in general, if you are against cutting funding for health research and public health, your representatives in Congress can do something about that.
• Dr. Komaroff is a physician and professor at Harvard Medical School. Send questions to AskDoctorK.com.