The growing number of viruses that have spun off from two main strains are creating a dynamic that is concerning, the global health agency said in an assessment of the evolving influenza situation.
"Virologists interpret the recent proliferation of emerging viruses as a sign that co-circulating influenza viruses are rapidly exchanging genetic material to form novel strains," the statement said.
"The consequences for animal and human health are unpredictable yet potentially ominous."
Infections with H5N1 bird flu in Egypt have soared since last November, with 108 cases and 35 deaths. That is more infections in four months than any country has reported for an entire year since the virus started to cause massive poultry outbreaks and sporadic human infections in Asia in late 2003.
Meanwhile, Egyptian authorities recently discovered an infection with an H9N2 bird flu virus. The patient — a toddler — recovered. But the fact H9N2 is circulating in close proximity to H5N1 viruses is worrying, said the WHO.
The concern with these and all animal flu viruses is that they reassort, swapping genes in ways that create new viruses. If one of these real world science experiments produces a virus that can easily infect people and pass readily from one person to another, a flu pandemic could occur.
"Influenza viruses can constantly reinvent themselves in a dazzling array of possible combinations. This appears to be happening now at an accelerated pace," the WHO said.
There have been 777 known cases of H5N1 infection since late 2003 and 428 of those people have died. That is a fatality rate of 55 per cent.
Slightly less deadly is H7N9, a bird flu virus that emerged in China in late winter 2013. It appears to kill about 36 per cent of infected people. But the virus may find it easier to infect people than H5N1. In the two years since it first appeared, H7N9 has infected at least 602 people and 227 of them have died.
In the past two years, new bird flu viruses have popped up. There are now H5N2, H5N3, H5N6 and H5N8 strains.
All this activity comes at a time when the world's need for more effective flu vaccines has been firmly underscored by this winter's mismatch of the seasonal flu shot to the circulating human flu viruses, the WHO said.
H3N2 viruses caused most of the disease this flu season, but the version in the vaccine was out of date, rendering the vaccine poorly protective for most people.
The WHO update said that while pandemic preparedness levels in the world are higher than they have been in the past, better vaccines and vaccines that are faster to make need to be a global goal.
"During a severe pandemic, many lives will be lost in the three to four months needed to produce vaccines," the agency warned.
Follow @HelenBranswell on Twitter.