The virus most worrisome to some physicians these days is found, not in a petri dish, but on the Internet.

It concerns the growing number of Web sites, perhaps 40 or more by one account, where patients rate everything from a doctor's skill to how much time patients spend in the waiting room.

Perhaps more noteworthy is that physician rating sites have gone mainstream, with consumer review services such as Angie's List and Zagat starting their own ratings on medical care.

Now one doctors' group is fighting back with its own Web site and a unusual strategy to curb unflattering postings. Angie's List, for one, plans to respond in kind.

North Carolina-based Medical Justice, which was launched 10 years ago to fight frivolous malpractice suits, has begun encouraging doctors to require patients to sign a "mutual privacy agreement" before treatment.

By signing, the patient promises not to post critical comments about his or her care on the Internet without the doctor's permission, though it doesn't prevent sounding off to family, neighbors or the state medical board.

Consumer sites have a different name for the agreements: gag orders.

Angie Hicks, of Indianapolis, founder of Angie's List that publishes reviews for paying members on a range of household services, said this week that she intended to start noting in her ratings which doctors require the waivers.

"This really comes down to a trust issue," she said. "If someone asks you to sign this waiver, how much trust do you have in them?"

Angie's List added health care ratings a year ago because members demanded it, Mrs. Hicks said. The site requires that posters reveal their identities to Angie's List staff and the physician -- but not to other members -- to give the report more validity.

She believes that the ratings can help physicians.

"I've seen tons of providers really grow their business because it allows them to magnify the word of mouth," she said. "The providers who benefit are open to hearing criticism -- and criticism is part of the nature of doing business; they learn from it and make adjustments."

She sees the ratings simply as an extension of what's been happening for years: people discussing their doctors, their hospital stay, their dentist, their pharmacy and many other health matters important to their families -- only over the Internet instead of over the back fence.

"It's a dicey issue," said Leo McCafferty, a plastic surgeon based in Shadyside, "because freedom of speech runs really at the core values of our country, and I don't think any of us want to inhibit free speech.

"But, let's face it, a physician can't control what a patient may say. It behooves both the physician and the patient to be responsible."

An individual's primary care physician is the best source for deciding which physician to see, Dr. McCafferty said, and patients on their own can check to see if a doctor is board certified. Internet sites also can be part of the equation, he added, but, "There has to be some way to monitor or measure the credibility of these rating systems."

Jeffrey Segal, a retired neurosurgeon and founder of Medical Justice (www.medicaljustice.com), believes most of the 100 or so Pennsylvania physicians who are members are using the agreements, though he did not have specific numbers. A recent article in Florida Health News said up to 300 physicians in that state now require a signed agreement from patients.

Dr. Segal said it was a lack of oversight and accountability on the Web sites that troubles doctors. Earlier this year, a California dentist sued the parents of a boy she had treated after they posted complaints about her treatment that she said were untrue.

"People are free to post any type of commentary," said Dr. Segal. "Most of these sites have only two, three, four reviews on a physician. There's no verification that the person was a patient. They could be a disgruntled employee, an ex-spouse, a competitor -- anyone trying to create some havoc."

Online comments on physicians and hospitals have circulated for years. One site, Ratemds.com, has listings for 5,400 physicians in Pennsylvania, each marked with a smiley or frowny face or, less often, a neutral expression. Typically those images are based on fewer than three anonymous patient evaluations.

Dr. Segal argues that doctors have no way to challenge unfair or inaccurate ratings -- federal patient privacy laws prevent doctors from responding. The mutual privacy agreements, as yet untested in a courtroom, protect physicians from unfair or inaccurate criticism they can't counter, he said.

Mrs. Hicks said rules against anonymous postings on Angie's List -- which is more the exception than the rule among physician rating sites -- prevents most of the scenarios posed by Dr. Segal. "The provider sees who's reporting."

Angie's List asks members six or seven questions, such as whether the doctor gave clear explanations and whether the office staff was courteous, but Mrs. Hicks said the real value is the written commentary.

So far, they have reports on 65,000 physicians ("The majority of the reports we get are positive") and are receiving an additional 10,000 health-related reports each month.

Dr. Segal acknowledged that online ratings are here to stay, so his group is working on a site of its own that will require verification that the poster actually was a patient of the physician, that a minimum of 50 reports come in before a rating is posted, and that a medical expert back up a patient's criticism of the medical care.

With those protections, he said, "I think we can get physicians to buy in."