Federal health authorities have signed a two-year deal to help states buy more than half a billion dollars worth of the antiviral drug Tamiflu as a hedge against a pandemic of deadly avian influenza, but there is a catch: States will have to pay for three-quarters of it. Under terms of the deal negotiated with Roche by the Department of Health and Human Services, the states can order up to 31 million packets of Tamiflu -- each containing a 10-pill course of treatment -- for a total cost of $596 million over the next two years. The Bush administration announced late Friday that it had contracted with Swiss drugmaker Roche Laboratories Inc. to supply Tamiflu for stockpiles in all 50 states. The federal government, meanwhile, plans to build its own centralized stockpile. The plan is to have enough antiviral drug in state and federal warehouses by December 2008 to treat 81 million people. Tamiflu is considered by scientists to be the first line of defense against the H5N1 strain of bird flu. The disease is currently confined primarily to chickens, ducks and some wild waterfowl, but researchers fear it could mutate into a form that spreads easily among humans.