From Fern's own wiki article:
The Senate began work on its own proposals while the House was still working on the Affordable Health Care for America Act. Instead, the Senate took up H.R. 3590, a bill regarding housing tax breaks for service members.[86] As the United States Constitution requires all revenue-related bills to originate in the House the Senate took up this bill since it was first passed by the House as a revenue-related modification to the Internal Revenue Code. The bill was then used as the Senate's vehicle for their healthcare reform proposal, completely revising the content of the bill.[