With more than 150 flu deaths in Mexico, and new cases in Europe and the U.S., health officials are scrambling to answer a basic question: Is this the Big One? The flu pandemic that ravages the world as in 1918, killing millions of young, otherwise healthy victims?

Or will it be the Big Fizzle, like the feared epidemic of 1976?

The unsatisfying answer: We won't know for a while.

Meanwhile, public health officials around the world wisely aren't taking any chances. They're mounting an aggressive effort to limit the spread of the virus.

Late last week, Mexican officials shut schools, museums and other public culture venues. On Sunday, American health officials declared a public health emergency to free resources, including flu vaccines, girding for a possible onslaught of the virus. On Monday, Homeland Security Secretary Janet Napolitano said the United States is preparing as if the outbreak is a pandemic. At the same time, European Union health officials urged Europeans to postpone non-essential travel to the U.S. and Mexico. Russia, Hong Kong and Taiwan said they would quarantine visitors showing symptoms of the virus. On Tuesday, Cuba suspended flights to and from Mexico, becoming the first country to impose a travel ban.

As fast as the virus travels, worry and fear travel even faster. But from all we know at the moment, there's no reason to panic. Let's take a deep breath (OK, you can use a mask) and remember a few important points:

n We really don't know how many swine flu cases are out there, or what the death rate is among those infected. That's an important piece of the puzzle for researchers. A typical flu bug kills only a tiny fraction of those who are infected, around one-tenth of 1 percent. But if the flu is more deadly, it might kill 1 percent or more. It is believed that the infamous 1918 flu killed about 2.5 percent of its victims. But scientists say that even if such a virus were to sweep the country, the death rate would likely be lower because of advances in medical treatment.

n We don't know why people are dying in Mexico but not in the U.S. Moreover, the strain in Mexico appears to be killing young adults, which resembles what happened in the 1918 pandemic. That's why researchers are so worried about this virus, which has been identified as a pig version of a human flu virus. Why would such a virus be lethal to otherwise healthy people? The theory: A virus essentially new to humans triggers a huge overreaction in healthy immune systems, creating what is called a "cytokine storm." If that happens, the lungs can fill with fluid and you can essentially drown "from the inside out," in the gruesomely memorable phrase of a reviewer of a 1999 book about the 1918 pandemic.

n We don't know how long this version of swine flu has been circulating. There are now cases in six countries, including the U.S.

So what can you do to protect yourself and your family?

The same precautions as always: Wash your hands, and keep them away from your face. If you're sick, stay home. If your child is sick, keep him or her home.

If you do get the flu, there is treatment available. There's no need to rush to the emergency room at the first sign of a sniffle. This isn't 1918. Tamiflu and Relenza appear to be effective, according to Dr. John Flaherty, an infectious-disease expert at Northwestern University's Feinberg School of Medicine.

"We are in the early phase of something that may turn out to be a false alarm or may turn out to be bad," he says.

The U.S. and other countries are taking smart precautions, gearing up in case of a pandemic. But until we know more, let's not spread an epidemic of fear.