Cases of norovirus, a nasty stomach bug that spreads easily, are climbing in the Northeastern U.S., the Centers for Disease Control and Prevention reported Thursday.

Nationwide, about 12% of most recent norovirus tests sent to the CDC were positive, but the proportion was about 16% in the Northeast, the agency said. That compares with nearly 10% of norovirus tests in the Midwest and South and nearly 13% in the West.

Characterized by the sudden onset of vomiting, diarrhea and general feelings of misery, norovirus outbreaks are notorious on cruise ships, nursing homes, jails, schools and other places where people are in close contact.

Here’s what you need to know about this wily germ:

WHAT IS NOROVIRUS?

Norovirus infections are caused by a group of viruses that spread remarkably easily. It can take as few as 10 viral particles — “a miniscule amount” — to make someone sick, said Dr. William Schaffner, a infectious disease expert at Vanderbilt University Medical Center.

HOW DOES NOROVIRUS SPREAD?

Norovirus can spread from person to person, in food or water or on contaminated surfaces. Because it’s so contagious, one handshake or a the touch of a contaminated door knob or handrail can be enough to cause illness, experts said.

HOW LONG DOES A NOROVIRUS ILLNESS LAST?

Illness caused by norovirus typically starts suddenly, in what Schaffner called “a strikingly dramatic way.” A person can go from slightly unwell to miserable within hours.

It usually lasts two to three days. Most people recover fully.

WHO IS AT RISK?

There is no medication to treat norovirus. Dehydration from vomiting and diarrhea is a chief worry, so those most at risk include young children, older people and those with weakened immune systems.

It’s important to replace fluids by sipping water, soda or other drinks — except coffee, tea and alcohol — during illness, Schaffner said. Anyone experiencing symptoms of dehydration should seek medical help, he added.

HOW CAN I AVOID BEING INFECTED WITH NOROVIRUS?

The best defense against norovirus infection, especially during the peak winter season, is rigorous and frequent handwashing. Use ordinary soap and warm water and scrub hands vigorously for 20 seconds before meals.

Cleaning surfaces is important, too. Use household disinfectants and scrub well, Schaffner said.

IS THIS SEASON WORSE THAN PAST YEARS?

The nationwide trajectory of norovirus doesn’t seem very different this year than in past years, but there are still a few weeks left in the winter, experts noted.

The unpleasant truth is that a norovirus surge is to be expected at this time of year, said Dr. Daniel Griffin, an infectious disease expert at Columbia University Medical Center.

“We often call it ‘winter vomiting disease,’” he said.