Epidemics Are Part of Nature . . .

Currently the world is riveted to the hourly news updates citing the rapid spread of swine influenza virus infections from Mexico to countries on at least five continents. The debut of this virus causes concern because of the global lack of immunity, the large number of deaths in Mexico, and its apparently efficient transmission from person to person.

Unlike the annual visitations with influenza, the current swine strain is attacking young adults, most of whom have no underlying diseases. At the time of this writing, there have been two deaths in the U.S., but nearly 180 deaths have been ascribed to swine flu in Mexico.

Influenza is an old virus whose primary reservoir is in migrating aquatic birds. From birds, the virus (1 billion viral particles per gram of guano in infected birds) can spread to pigs, which have the ability to be co-infected with two viruses at the same time. In the large airways and lungs of pigs, the co-infecting viruses can mix and match genes, creating offspring with viral markers of both pigs and birds.

SINCE HUMAN-adapted viruses can infect pigs also, the exchanges of genes can create mixtures of avian, human, and swine elements. The new swine flu virus of 2009 -- although fundamentally a pig virus -- has in fact genetic elements of influenza viruses of pigs, birds, and people.

In historical perspective, what worries health authorities is whether or not this virus will create a deadly pandemic. A pandemic is a sustained, international transmission of infections. The 1918-19 pandemic of influenza ("Spanish flu") had a 2.5 percent mortality rate, whereas our seasonal influenza has a mortality of 0.1 percent (25-fold less). The lesser 20thcentury pandemics in 1957 ("Asian flu") and in 1968 ("Hong Kong flu") had a mortality approaching 0.5 percent -- very serious but still five-fold less than that in 1918-19. We have not had any influenza pandemics since 1968.

The first swine flu cluster of infections in the U.S. occurred in February 1976 at Fort Dix, N.J. Two hundred thirty young recruits were infected, 13 developed severe respiratory infections, and one man died. Although a few other sporadic cases were reported, essentially no generalized epidemic occurred. Nevertheless, President Gerald Ford in March of 1976 announced a national vaccination program, having been influenced by health policy analysts.

When the vaccine was delivered, an unexpected neurological side effect occurred causing weakness and paralysis (the Guillian Barré Syndrome) at a rate seven times greater than that seen in usual flu vaccine recipients. Five hundred people who received the vaccine developed the syndrome, and five died. There was a shakeup at Centers for Disease Control subsequently, and for years the public's willingness to accept any vaccine program was diminished.

THE CURRENT swine flu virus is different from the one in 1976, especially since it is related to transmissions to people in many countries. The initial infections occurred in Mexico, and international travel facilitated global spread. What is uncertain is whether there will be a series of new waves of infection in each country, and if the death rates in countries outside of Mexico will rise.

In 2009, we are fortunate to have two drugs that are active against the new swine flu virus. Both would be expected to be very good for preventing infection among those exposed but not yet ill. Each, however, is only modestly effective as a therapeutic drug, and neither has been tested in life-threatening situations. The latter is a strong reason why a safe and effective vaccine is being sought.

It is unclear whether swine flu will act more like the devastating avian flu pandemic of 1918-19, the serious but less severe pandemics of 1957 or 1968, or our less serious annual flu epidemics. However, the latest "influence" to visit itself upon the citizens of the world reminds us that epidemics are part of the natural interaction of people and microbes throughout civilization. Yet each time we react as though we have discovered something new.

The only really new things are our surprise and consistent inability to recall all of the lessons from prior visitations.

Dr. Richard P. Wenzel is chairman of the Department of Internal Medicine at Virginia Commonwealth University, immediate past president of the International Society for Infectious Diseases, and a member of the Richmond Academy of Medicine. Contact him at . Dr. Michael B. Edmond is professor and chairman of the Division of Infectious Diseases at VCU and hospital epidemiologist at the Virginia Commonwealth University Medical Center. Contact him at .