The recent media commotion over the Mexican Swine Flu is another case of sensationalism overwhelming fact. Early on, this new strain of flu was touted in the media as the possible beginning of a worldwide killer plague (or pandemic). While this was, early on, a possibility, it quickly became apparent that this was not the killer flu the headlines were screaming about.
Influenza has been around for thousands of years, and developed in Eurasia as a result of people living in close proximity to domesticated animals (typically chickens and ducks, or pigs). The animals have an upper respiratory (from the throat to the lungs) infection that gets passed to humans. Normally, passing a viral disease species-to-species has no effect. But viruses tend to adapt rapidly via mutation, and every once and a while, this species-to-species passage of the influenza virus results in a new strain that makes humans sick. These strains of influenza, because they are so unfamiliar to the human immune system, do more damage, and are more frequently fatal. Even without going species-to-species, influenza viruses are constantly mutating, and some years there are strains that are more likely to kill.
Every few centuries you get a really bad strain, like the one that showed up in 1918. This one infected about a third of the population, and killed ten percent of those infected. Some 2-3 percent of the entire human population died. This was the first really nasty strain that spread worldwide, because of steamships that had been developed in the late 19th century, and were now constantly moving across the entire planet. This was an advantage influenza never had before, as in the past, a really bad strain would mutate and die out before it could do a world tour. Thus, in a manner of months, the 1918 (or "Spanish") flu spread to every corner of the planet. Normally, each year the latest mutation of influenza infects 5-15 percent of the population, killing one in a thousand of those infected (versus one in ten for the 1918 flu). Currently, the normal flu kills 30,000-40,000 people a year in the U.S., and 300,000-500,000 worldwide.
The last "killer flu" was in 1957-58, and it infected 25 percent of those exposed, killing about one of every 650 who got it. The 1968-69 flu was designated a "pandemic" but it was only about 25 percent worse (in terms of fatalities) that the usual annual flu. The media keeps alive the threat of another 1918 scale pandemic, but such events are very rare (except to those looking for a hot headline.)
Most importantly (and much less newsworthy), we are much better prepared to deal with a super flu, than was the case in 1918. This is especially true when you consider that most deaths due to pandemic flu were not to the flu itself, but to other infections (like pneumonia) that more much more dangerous because the victims had been greatly weakened by influenza (which eventually passes, leaving the victim with more immunity to flu in general). In the 1940s, antibiotics (like penicillin) were invented, and these drugs can quickly treat most diseases that killed weakened flu patients in 1918. But a super flu is still deadly in many parts of the world that do not have access to modern medicine (about half the world population is thus vulnerable).
If the recent Mexican strain of the flu had been 1918 grade, we would have known it very quickly, just because of the high body count. So far, it has only proved fatal in Mexico, probably because it is hitting poor people without regular access to medical care. It's also possible that the few American tourists who returned home with it, had an already mutated strain that was weaker. The mutations go on all the time, and either make the flu less, or more, deadly. The American strain appears to be less deadly, about on a par with the strain that sweeps the nation each year.
But there is a danger. Influenza, despite its constant mutation, tends to retain one critical characteristic; it spreads most efficiently in cold weather. That's why "flu season" is in the Fall and Winter. A strain that infects a higher proportion of the population (like the 1918 version), will spread in warm weather, and ravage tropical countries (where most of the population has little exposure, and immunity, to the flu in general.) If the current Mexican strain does mutate into a more lethal form, it could take off in six months (in the northern hemisphere) and do some real damage. But that seasons delay gives drug companies time to develop and manufacture a vaccine. No vaccine provides total protection (because of the constant mutation), but a vaccine against the Mexican strain would greatly reduce the impact of the disease. At least in wealthy countries, where the health system can get the vaccine to a generally healthy population. Most of the planets population (in poor nations with less healthy inhabitants) could still be in for a hard time. But not as deadly if there were no vaccine and modern medicines available.