Depends on when this takes place, in thirty years the US will be a third-world country after massive depression b/c every person lives paycheck to paycheck meaning the consumer is broke. Its already a tenuous grasp one step away from being homeless or maxing out their credit card. Once the crunch happens, the credit cards shut down b/c of the economy and no one can pay their bills.
The workforce is one third retired, less workers meaning more higher wages meaning the elderly cant afford to pay for goods and services on the miniscule social security and the vast majority have not managed to save a penny.
Massive elderly entitlements b/c they vote taxing the hell out of the workers. Companies move to China to escape the tax burden.
All of the US collapses, and then comes..... the zombies. To mop up after chaos has ensued. Elderly population ripe to eat their flesh. Zombies flourish and overpopulate. Move to Mexico and enter South America.
Deathly afraid of water, they are contained to the Americas until a genetic mutation in a sect of the population making them hydrophyllic instead of hydrophobic. Suddenly, the Zombies becomes a global pandemic, able to operate large ships with their new "sea legs wearing sea genes" they cross the ponds in both directions.
First stop they land at Normandy of all places. France immediately surrenders and is eaten or becomes zombies themselves only to move across the English Channel and then eastward expansion takes place. to be cont...