It can begin with explosive suddenness, what a St. Joseph Medical Center infection control specialist called "an aggressive evacuation of fluids from the body," followed by days of weakness and malaise.
It's the norovirus, a gastrointestinal infection often mistakenly called the stomach flu. A common affliction in winter, it has hit Marylanders especially hard in recent weeks, according to public health officials.
Last month, St. Joseph hospital was seeing an average of 10 patients a day in the emergency room, "and maybe two to four were admitted with severe dehydration," said Leigh Chapman, manager of infection control there.
"Sometimes families would come in; it would attack the whole family," she said.
And the season may not have peaked yet, according to the state Health Department. "We can't ever know when the peak of the season is until the season is over," said Alvina Chu, chief of the department's division of outbreak investigations.
The norovirus is the same type that sometimes infects cruise ship passengers, causing severe gastroenteritis to sweep through the onboard community.
Severe symptoms can appear just 12 to 48 hours after exposure and may last 24 to 48 hours. They include diarrhea and vomiting, stomach pain, low-grade fever, headache, muscle aches, chills and exhaustion.
The Health Department estimates the severity of the norovirus season by counting "outbreaks" (three or more closely related cases within a week, typically in institutions such as hospitals, schools or nursing homes). It investigated 32 Maryland outbreaks during the middle three weeks of December, compared with just three during the same period in 2007. Chu said that's probably because this year's norovirus season began earlier than usual.
The greatest danger from the illness is dehydration, especially among the very young or the elderly who have other medical problems, such as diabetes. Severe fluid loss can require treatment in a hospital.
"That can make patients feel so weak. ... It knocks you out," said Chapman, who also fell ill with the virus. She compared its acute effects with those of food poisoning.
The best way to prevent a norovirus infection is careful hand hygiene, health officials agree.
"It's just so important," especially after being in public or using communal phones or keyboards, Chapman said. "You want a balance of washing your hands, whether it's with soap and water or alcohol-based sanitizers; and respiratory etiquette. Do not cough or sneeze into your hand; cough into your arm or sleeve."