How can we be sure how serious the threat is? We can’t.

According to the World Health Organization, which is carefully tracking the new H1N1 virus and reporting daily, as of 4 PM today (Wednesday, May 6), 23 countries have reported 1,893 official cases (822 in Mexico) and a total of 30 deaths (29 in Mexico). Since I wrote on this last week, the number of official cases, the number of countries, and the number of U.S. states that have cases have all doubled two or more times (due partly to the spread of the virus, partly to the spread of laboratory facilities competent to identify it).

But the number of deaths has paradoxically declined, because most of the deaths reported in Mexico seem to have been from other causes. The case fatality rate, which is the number of deaths divided by the number of cases, has declined even more. It was overestimated when the epidemic was limited almost entirely to Mexico because most of the cases that came to light were severe. In other words, the denominator in the fraction was underestimated. In fact, as the epidemic has unfolded, the vast majority of the cases have been mild, something like the average severity of the seasonal flu.

Am I sorry I took steps to protect myself and my family? Certainly not.

I wrote last week, “I don’t know why this new Mexican swine flu tipped the balance for me. I don’t give it more than a one percent chance of becoming the kind of challenge that would drive us into the basement to draw on those stores of long-grain rice and black-eyed peas. But then, I don’t give myself more than a one percent chance of dying within the year, yet my life insurance premium gets paid every month.”

Richard Besser, the acting director of the Centers for Disease Control, said the following on Meet the Press on Sunday: “We’re starting to see some encouraging signs…and that’s good. But our approach has to be aggressive. With a new infectious disease, you may only get one chance to get out in front of it, and that’s what we’re doing.”

By the same logic, the WHO raised the pandemic alert level from phase 4 to 5, phase 6 being a full-blown pandemic. Now, a pandemic doesn’t have to be severe to be official, nor does it have to cause many deaths, it just has to be worldwide and affect a certain number of countries. We may or may not get to phase 6 this spring, at least in the northern hemisphere, where most of the human species lives.

This is because influenza dislikes hot weather, and we northerners are coming to the end of the season when it can thrive. The southern hemisphere is a different story, and it is on very high alert for possible spread of H1N1 during the coming winter in July and August.

But whether or not it spreads or worsens below the equator in the next few months, there is much reason to think we are not done with it. It will lie in a sort of epidemiological dormancy until our winter comes next year, and then it could return with a vengeance.

In the meantime, unfortunately, it seems we may have to choose between making large amounts of a vaccine against this new virus, and making the usual vaccine against seasonal flu, which we know kills scores of thousands of people every single year.

So why even take this new virus seriously? Because it is new. Because we don’t know what it can or will do. Because it has certain things in common with a virus that in 1918 killed a minimum of 20 million people. Because it will very likely come back again next year. It’s the restless tide of the microbial world lapping at the shores of the human future. We just have to face the fact that next year we will have two influenza enemies, not one.