For all that scientists have learned about influenza since the catastrophic pandemic of 1917-19, one thing has not changed: the predictably unpredictable nature of the viruses that cause it.

The sudden detection of the new swine influenza virus, A(H1N1), occurred just as scientists were focusing wary eyes on behavioral changes observed in another virus, the A(H5N1) bird flu strain, in Egypt.

The World Health Organization said on April 30 that a global spread of swine flu was likely and raised its pandemic alert level to Phase 5, the secondhighest level. Thus far, however, the epidemic has resulted in mild illness in many of those infected outside of Mexico, which is considered the epicenter of the outbreak.

The W.H.O. and public-health agencies find themselves in a delicate balance, obliged to provide information about potentially lethal diseases and prepare for the worst without causing panic. If the alarm is sounded too loudly, officials will be ridiculed for unnecessarily worrying millions of people. If a pandemic materializes, some of the same critics are likely to blame officials for failing to prevent it.

The W.H.O. recently advised against travel and trade restrictions, saying it was too late to contain the virus. President Barack Obama concurred, saying it would be“akin to closing the barn door after the horse is out.”

History teaches that the influenza virus mutates to cause worldwide spread about twice a century, on average. But scientists have yet to figure out what causes the mutations, when they will occur and what makes certain viruses more lethal than others.

High initial death rates for a new or mutated virus often fall as officials find that the causative microbe also causes mild, even symptomless cases.

H1N1 appears to be passed easily from person to person, and reports from the United States suggest that some cases may be mild and therefore may go undetected. As of May 1, the W.H.O. reported 331 confirmed cases in 11 countries, with 15 deaths in Mexico and one in the United States.

In contrast, avian flu has killed 257 of the 421 people who have contracted it. But it has shown little ability to pass from person to person, mainly infecting poultry.

SARS, or severe acute respiratory syndrome, is easily spread and virulent. In a 2003 outbreak in Hong Kong, it killed 299 of the 1,755 people it infected.

Scientists fear that the swine flu, which contains genes from humans, birds and pigs, may pick up more human genes as it spreads and become more dangerous. But they have been wrong before.

In 1976, after a small outbreak of swine influenza at Fort Dix in New Jersey, public-health officials persuaded President Gerald R. Ford and Congress to mount a nationwide immunization campaign that came in for widespread criticism. Yet 60 years earlier, an influenza virus that apparently started as a mild outbreak in the spring came back in a giant storm months later.

Which model will the swine influenza virus follow? Scientists can only prepare for the worst and hope for the best.