College Park recently experienced two major outbreaks of the mysterious norovirus, a violent, fast-moving gastrointestinal illness. The first, in early August, infected about 100 teenagers visiting the University of Maryland for a youth leadership conference. The second, a few weeks later, laid low about the same number of people at the nearby American Center for Physics. In recent years, the flulike ailment has also been reported in hotels, nursing homes and schools in Northern Virginia. Staff writer Amy Argetsinger talked about the virus with Michael S. Donnenberg, a professor of medicine and head of the Division of Infectious Diseases at the University of Maryland at Baltimore.

Q What is the norovirus?

ANorovirus is a very small virus that causes gastroenteritis in humans, and humans are the only known hosts of this class of viruses. It's an illness that usually begins abruptly with nausea, vomiting and diarrhea, and there can be a low-grade fever. It's a short-lived infection -- you're usually only sick for a couple of days -- but when outbreaks occur, they're very dramatic. Twenty-four hours after everyone's together, about half the people get sick. There's no specific therapy; the main thing is to maintain hydration.

How is it transmitted?

It appears that the number of virus particles you need to ingest to get sick is very small, and it's excreted in very high numbers, so there are very many ways you can get it. You can get it from direct contact with someone who is ill; from eating food that is contaminated, usually in preparation by someone who has had the virus; you can get it from contaminated water; you can get it from contaminated surfaces. We even think you can get it from aerosol.

When you say aerosol, do you mean from sneezing -- from inhaling sneezed particles?

Actually, from vomitus. It gets aerosolized, and you can get it that way.

Is that unusual for an infectious disease?

Very.

Norovirus has been in the news a lot the past two years, but you hardly ever heard about it before then. Why is that? Is it something new?

The virus was discovered in the 1970s, and at first the only way to detect it was electron microscopy, which is cumbersome. The virus can't be cultured in tissue culture, so it wasn't until they cloned the genome that they were able to develop better tests. Now there are rapid tests available that are based on the genetic material of the virus. I don't think it's new. It's the most common cause of gastroenteritis. It's just that most cases of gastrointestinal disease go undiagnosed. Diagnosis is more common when there's an outbreak situation. . . . It has a tendency to recur in particular settings, such as on cruise ships or institutions, and that's newsworthy.

The appearance of two cases so close to the University of Maryland led many to wonder if there was a connection between the two. Is that likely?

[Norovirus] is very common all over the country. Those two outbreaks close in time and space suggest that possibility, but it's difficult to pin that down. There are different strains that can be detected by molecular technique, and if that were done, it could rule out a connection. But people can be asymptomatic -- they can carry it and infect others -- so it's possible there was a common chain of transmission between the two outbreaks that was unappreciated.

MICHAEL DONNENBERG