The WHO's annual assembly opening on Monday is set to be dominated by swine flu with infections across the globe having soared above 8,000 and Japan closing hundreds of schools to battle an outbreak.
The A(H1N1) virus has already had an impact on the meeting of 193 member states: the WHO has proposed shortening the gathering from 10 days to five to minimise the time senior health officials are away from their countries.
The UN health agency has said they are needed back home to coordinate national efforts against swine flu, which has swept into at least 39 countries in less than a month.
A committee will take up the proposal to shorten the meeting on Monday. If it is endorsed, planned discussions on tuberculosis, viral hepatitis or human organ transplants could be dropped.
Since it was brought to world attention in Mexico and the United States, the new swine flu virus has infected nearly 8,500 people, according to WHO, and spread beyond North America with travellers.
The WHO raised the global flu alert to level five on April 29, one step away from outright pandemic or sustained spread of the disease on more than one continent.
Japan's number of confirmed swine flu cases soared to 93 at the weekend, officials said late Sunday.
Most of the infections were reported among high school and college students in and around the western cities of Kobe and Osaka, where authorities ordered more than 1,000 schools and kindergartens to stay shut on Monday.
More than 70 people have died from the virus, all of them in the Americas and nearly all in Mexico, where the new strain of swine flu was first detected.
Chilean health officials on Sunday reported the country's first case, which would make it the 40th nation affected.
High-level discussions were expected to be held on swine flu at the WHO on Monday ahead of an official appearance later in the day by WHO Director General Margaret Chan. UN chief Ban Ki-moon is due to appear on Tuesday.
Hours of debate in the annual meeting are expected to be devoted to influenza and pandemic preparations, a regular topic in recent years after H5N1 bird flu took hold in humans.
At a meeting in Geneva ahead of the assembly world governments failed to reach a final deal on the sharing of virus research material and vaccines in case of a global flu pandemic.
The WHO is also holding talks with scientists and vaccine makers about triggering large scale pandemic vaccine production and whether to halt seasonal flu production in order to free up production capacity.
Experts have also expressed concern about possible mutation of the A(H1N1) virus that could make it more resistant to antiviral drugs such as Tamiflu and Relenza, which have been stockpiled by countries to treat severe flu.
The assembly is also due to examine the implementation of International Health Regulations, which since 2005 oblige countries such as Mexico to swiftly raise the alarm about potential international health threats.