Poorer nations, especially in the southern hemisphere which are heading into their winter season, face the greatest risk from the flu pandemic, experts warned.
Developing countries have fewer resources and could suffer "a huge impact" from the A(H1N1) virus, according to Adam Kamradt-Scott, research fellow at the London School of Hygiene and Tropical Medicine at the University of London.
"They don't have the health-care infrastructure, they don't have the resources to buy anti-viral medications and potentially... they may not have the money to buy vaccines if some were to be developed," he said.
A(H1N1) virus has already spread across the affluent world -- from North America through Europe to China, Japan and Australia.
Affected countries have taken action to isolate patients, ordering the closure of schools and entertainment venues in a bid to stop the spread.
However, experts fear that the disease will be harder to contain if it reaches developing countries.
A 2006 study led by Harvard University professor Christopher Murray, based on mortality during the Spanish flu disaster, estimated that 62 million people could die in 12 months if a similarly lethal pathogen emerged today.
Ninety-six percent of those deaths would occur in low-income and middle-income countries.
UN Secretary-General Ban Ki-moon warned recently that developing nations are "especially vulnerable" from the new virus.
"If we are indeed facing a pandemic, we need to demonstrate global solidarity," he said. "In our interconnected world, no nation can deal with threats of such dimension on its own."
Experts believe swine flu would taper off in the northern hemisphere as summer sets in but the southern hemisphere is at greater risk as it heads into winter.
Hot temperatures would not help propagate the virus, noted Patrick Berche, who heads the Necker children hospital's microbiology section in Paris.
"Therefore, if there's a pandemic, the virus will be confined to the southern hemisphere which would be in winter and could return," he said, speaking before the WHO declared a pandemic on Thursday.
WHO acting director-general of Keiji Fukuda said experts are watching developments in the southern half of the world very carefully.
"It's possible we will see outbreaks of the H1N1 virus occurring more recently in the southern hemisphere than in the northern hemisphere. This is something we have to be on the watch out very carefully for," he said.
The WHO has stressed that all regions would eventually be at risk in a pandemic.
"I think that in this age of global travel where people move around in airplanes so quickly, there is no region to which this virus could not spread," Fukuda said recently.