Today it's Ebola Americans are worried about.  Two centuries ago it was Yellow Fever. In 1793 a Yellow Fever epidemic struck Philadelphia, costing the lives of some 5,000 inhabitants -- 10 percent of the population.  Two years later an outbreak in New York City took the lives of more than 700.  Then, in 1798 the three largest cities in the young republic -- New York, Boston, and Philadelphia -- simultaneously suffered from the disease.  Another five thousand lives were lost.

That December, for the first time, a president of the United States took official notice of the outbreak.  In his Second Annual Address, President John Adams opened with the good news that the "malady has disappeared and that we are again permitted to assemble in safety at the seat of Government for the discharge of our important duties."  But he warned that federal action was required to prevent a recurrence:

[W]hen we reflect that this fatal disorder has within a few years made repeated ravages in some of our principal sea ports, and with increased malignancy, and when we consider the magnitude of the evils arising from the interruption of public and private business, whereby the national interests are deeply affected, I think it my duty to invite the Legislature of the Union to examine the expediency of establishing suitable regulations in aid of the health laws of the respective States; for these being formed on the idea that contagious sickness may be communicated through the channels of commerce, there seems to be a necessity that Congress, who alone can regulate trade, should frame a system which, while it may tend to preserve the general health, may be compatible with the interests of commerce and the safety of the revenue.

Congress didn't need much convincing.  In February, just three months later, the legislature passed the first quarantine act. The law required federal officials to help implement state and local quarantine regulations.

The word quarantine -- French for forty days -- dates to the middle ages.  Passengers arriving by ship in Venice in the 1300s from ports where an outbreak had occurred were required to remain off-shore for a specified time period.  Other European coastal cities quickly adopted the same approach.  Originally it was believed that thirty days was sufficient; eventually, the authorities in European cities settled on forty, though it's not known exactly why.