TORONTO - The World Health Organization is looking for evidence swine flu viruses are spreading in sustained waves before moving to pandemic alert level Phase 5, the agency's top influenza expert explained Tuesday.

Dr. Keiji Fukuda said the WHO and its panel of expert advisers want to see that swine flu viruses haven't just spread to a country like Canada or the United States from Mexico in a traveller, but that they have actually taken root and are continuing to transmit.

"What we are looking for also are cases in outbreak which indicate that the infection is becoming established in a community or becoming established in a country," Fukuda, the WHO's acting assistant director-general for health security and environment said during what has become a daily media briefing.

"So even though we know that the virus has reached the United Kingdom and New Zealand, for example, in the form of infections in travellers, this is still a different situation than the infection becoming established in a community in those countries."

"And so this is a distinction that is important epidemiologically and important for everybody to understand," Fukuda said.

While it might not be clear to people not steeped in the study of infectious diseases, what that means is that the WHO wants to make sure these swine flu viruses are showing they have what it takes to continue to spread in the way human flu viruses do.

Some pathogens that move from animals to humans can cause a single infection, or trigger a generation or two of additional spread. So a person gets sick and infects another person, who infects a third.

But transmission can peter out, because some animal viruses aren't well adapted to humans.

The WHO has had years of experience watching for that kind of transmission pattern with H5N1 avian flu viruses.

The viruses, which are endemic in poultry flocks in parts of Asia and Africa, occasionally infect people. In a number of cases in the years they've been spreading, H5N1 viruses have triggered one or two generations of human cases.

But to date those viruses have only been seen to be able to spread through three generations -- so person-to-person-to-person -- before petering out. And that level of spread has been rare.

If disease investigators saw confirmed illness in people who hadn't been to an affected country but had contact with someone who got infected there, that would be of interest. If the new case was in someone several generations of contact away from an infected traveller, that would be a sign the viruses were becoming established in a community.

"What we've seen for the most part outside of Mexico are imported cases," said Dick Thompson, a spokesperson for the WHO. "These are largely visitors who've returned home and they've become sick."

"What we're looking for are patterns in which this imported case infects somebody else. So they go home, they're cared for by their husband or wife, that person becomes sick, that person infects the grocer, somebody goes to the hospital, health-care workers are infected. And then the health-care worker goes home and infects their family."

"This is intergenerational community spread. And we're looking for a sustained pattern to that."

WHO says that would signal that the swine flu virus is well adapted to human spread and poses an even greater pandemic threat.