In late 1776, as Gen. George Washington led his troops through the opening battles of the American Revolution, it was not necessarily the enemy fighters who posed the biggest risk to the fledgling U.S. Army.

An estimated 90% of deaths in the Continental Army were caused by disease, and the most vicious were variants of smallpox, according to the U.S. Library of Congress.

That's why Gen. Washington made the controversial decision to order the mass inoculation of his soldiers, an effort to combat spread of the disease that was at the time a major deterrent to enlistments and posed the risk of debilitating his army and tipping the balance of power against America's first warfighters.

According to the U.S. Library of Congress's Science, Technology, and Business Division, the smallpox inoculations began Jan. 6, 1777, for all of Washington's forces who came through the then-capital of Philadelphia, and through Morristown, New Jersey, following the Battle of Princeton.

Smallpox is a potentially fatal disease that starts with fever and vomiting and an outbreak of ulcers in the mouth and a skin rash. The skin rash turns into highly contagious fluid-filled blisters. The fatality rate was very high.

Inoculations were far more primitive - and dangerous - than today's vaccinations. The most common method was to cut a person's skin and rub the minor incision with a thread or cloth contaminated with a less-virulent version of smallpox, which in this case was a strain known as "variola."

At the time, most English troops were immune to variola, and their immunity gave them an "enormous advantage against the vulnerable colonists," according to the library. By contrast, less than a quarter of the American colonial troops had ever had the virus.

Washington knew a mass inoculation campaign could backfire and might cause more disease than it prevented. He also feared the mandatory inoculations would harm recruitment.

Nevertheless, after weighing the odds, Washington informed Congress on Feb. 5, 1777, of his plans for a mass inoculation. The general's plans contraindicated a 1776 proclamation by the Continental Congress prohibiting inoculations.

A Feb. 6 letter to Dr. William Shippen from Washington states: "Finding the smallpox to be spreading much and fearing that no precaution can prevent it from running through the whole of our Army, I have determined that the troops shall be inoculated. This expedient may be attended with some inconvenience and some disadvantages but yet I trust its consequences will have the most happy effects. Necessity not only authorizes but seems to require the measure, for should the disorder infect the Army in the natural way and rage with its usual virulence, we have more to dread from it than from the sword of the enemy."

Throughout February, the inoculations across the entire force were carried out in the model of the initial efforts in Morristown and Philadelphia.

Washington's strategy was largely successful.

"The isolated infections that sprung up among Continental regulars during the southern campaign failed to incapacitate a single regiment," the Library reported.

You can read more on the first mass military inoculation at the Library of Congress's Science section.