The fourth threat level represents "Evidence of increased human-to-human transmission." Level 6 is the highest level on the scale and indicates "efficient & sustained human-to-human transmission," or that a pandemic is taking place.

Dr. Keiji Fukuda, the World Health Organization's temporary assistant director-general for Health Security, advised nations to strengthen preparations because the virus is too widespread to make containment feasible. He said closing borders and imposing travel restrictions is unlikely to slow the spread of the disease.

"A pandemic is not considered inevitable at this time," he said. "The situation is very fluid and the situation continues to evolve."

After the announcement, stock market futures fell to about 0.7% below closing levels.

Fukuda said it may take 4-6 months to develop a vaccine and make the first batch available. It would take several more months to produce large quantities.

Fukuda said that more data is still needed. Some cases of the virus appear to be extremely virulent -- 103 people in Mexico are believed to have been killed by the disease -- while cases in the United States and elsewhere have been mild. Fukuda said the World Health Organization still doesn't understand why the virus has been so severe in Mexico.

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