Diseases are also unpredictable and even the experts are often wrong.
In 2003, Severe Acute Respiratory Syndrome (SARS) spread to 29 countries and caused nearly 800 deaths and many predicted a pandemic. That global scourge and the bird flu epidemic never materialized.
On the other hand, so-called mad cow (Creutzfeldt-Jakob) disease was under-played. With a long incubation period and spread through infected beef, the disease killed 165 people in Britain and six elsewhere before the public saw it as a real public health threat.
In a British pub with friends in 1993 while researching his book, Klitzman noticed 11 people at his table ordered the roast beef and Yorkshire pudding.
"I asked, 'Aren't you afraid?'" he asked. "They told me, 'No, the government says it's fine.' It had only killed a million cattle and house pets had died. But there was no sense that anyone wanted to talk about. I ordered chicken and they laughed at me."
"People don't want to worry about it until there is a law," he said. "We don't worry about seat belts until the cop pulls us over and gives us a ticket."
And public fear can fade as quickly as it swells. At the height of the HIV/AIDS epidemic of the 1980s, some refused to touch patients and shunned them like those with leprosy.
The late conservative columnist William F. Buckley once suggested that those who are HIV positive be tattooed.
But today, a recent Kaiser Family Foundation study found that Americans are least concerned about that disease than at any time in the last decade, even as the number of new cases rises.
As folklore suggests, public health can be as threatened by malaise as by panic when experts "cry wolf" too many times.
"When you see the wolf coming in to town, you want to have an instantaneous reaction," said Klitzman. "It used to be when the wolf came, you saw a wolf. Now you don't know how to react."
As for calculating an individual risk, he suggests dying of swine flu is an unlikely event. He advises periodic hand washing, getting enough sleep and drinking lots of fluids.
If flu symptoms emerge, "catch it early," he said, and get a prescription for Tamiflu.
"You have to put your risks in perspective," said Klitzman. "The family in Queens whose kids are sick, I hope they are being very careful. But at the moment, to say you shouldn't take an airplane because there might be someone next to you and you'll get sick is an overreaction."
"You are more likely to die today in a car or from smoking," he said. "You have to make decisions intelligently."