The invitation marks a breakthrough in Taiwan's long campaign for more participation in global groups, and comes amid a warming trend in cross-strait relations.
But the island's pro-independence opposition slammed the government for accepting a downgrade to Taiwan's sovereignty as the price of admission.
"The scope of Taiwan's participation will be decided by Beijing – it has the final word," says Lo Chi-cheng, a political scientist at Taipei's Soochow University. "So in a way, Beijing is the central government and Taiwan is like a local government. That's the problem we're facing now."
Taiwan will participate under the name "Chinese Taipei" – the awkward moniker it also uses at the Olympics – in a nod to Beijing's sensitivities.
Rather than attend as a member country, it will be as something short of a nation – part of a motley group of "observers," which in the past have included the Palestine Liberation Organization and the Knights of Malta.
Critics noted that the invitation was only for this year's meeting, and only on Beijing's say-so. That means China could block Taiwan's participation in future years if it was displeased with Taipei, or if the pro-independence party returns to power.
China has long boxed Taiwan out
Taiwan had pushed without success to join the WHA – the annual meeting of the top decisionmaking body for the United Nation's World Health Organization – since 1997.
Statehood is not a requirement for WHA observership. But Beijing had blocked Taiwan's participation anyway, insisting it represented the island at the world forum and could take responsibility for its health matters.
China sees self-ruled Taiwan as part of its territory, and has threatened war to prevent it from formalizing its independence.
Only 23 countries – mostly small and poor – recognize Taiwan as a nation, and it has been shut out of all United Nations activities since Beijing took over the "China" seat in 1971, and Taiwan was forced out.
Good timing – as swine flu spreads
Taiwan's participation at the May 18 to 27 assembly will be timely, coming during the global swine flu health crisis.
"Now, Taiwan can directly exchange health and epidemic-fighting information with the WHO, and more effectively safeguard our citizens' health and security," the government said in a statement.
Taipei has longed complained that being shut out of global health bodies and networks has put its citizens' health at risk.
In the early stages of the 2003 SARS epidemic, China blocked Taiwan from receiving health assistance and information, Taiwan's former government said, possibly costing lives.
In 2005, Beijing and the WHO agreed to allow Taiwan "meaningful participation" in the organization's technical meetings. But that memo specified that any such participation must be approved by China – another bone of contention here.
Early this month, Taiwan was included in a WHO information-sharing network, also thanks to China's approval. The island recently tapped into that network to get the latest global updates on the spread of swine flu.