Because they work with sick people every day, doctors and nurses are among the most vulnerable to the flu.

Many get vaccinated before flu season arrives to avoid getting sick or spreading the virus to their patients unintentionally.

But not every medical professional will get the shots for the common flu or H1N1 virus this year, and in most of the U.S., they don't have to.

The reason lies at the heart of U.S. federal law — the Constitution .

Under the 10th Amendment, powers not given to the federal government "are reserved to the States."

Over the years, that has been interpreted to mean that the federal government cannot require anyone to get vaccinated, according to Alexandra Stewart, director of the Epidemiology of U.S. Immunization Law project at George Washington University's School of Public Health and Health Services. 

Nor can Congress require states to mandate vaccinations.

"They can encourage, they can advise, but they cannot require states to do anything," Stewart said.

Some have argued even the states do not have the right to mandate vaccinations.

In the early 20th century, a Massachusetts resident sued the state for fining him $5 after he refused to comply with a smallpox vaccine mandate, arguing that it infringed upon his civil liberties. But in 1905, the U.S. Supreme Court ruled that vaccination mandates were a legitimate use of the state’s police power to protect its citizens’ public health and safety.

Since then, many states have required health care workers to get vaccinated against measles, mumps, rubella and other infectious diseases.

Yet no state required flu shots until this August, when the New York state health commissioner mandated that all health care personnel who have contact with patients be vaccinated against the seasonal flu and H1N1 or face fines, unless they had an exemption for medical reasons.

In response, dozens of New Yorkers protested the mandate at the state capitol in September and some groups — including three Albany Medical Center nurses — filed suits against the state, arguing that the mandate violates their 14th Amendment rights.

"The commissioner is acting in excess of his authority," said Terence L. Kindlon, a lawyer for the three nurses. "My clients are being told that if they don’t do this thing … that they will be fired from their jobs — and that's preposterous. That violates their civil rights."