Even as it has become increasingly clear that the bird flu outbreak on the nation’s dairy farms began months earlier — and is probably much more widespread — than previously thought, federal authorities have emphasized that the virus poses little risk to humans.

Yet there is a group of people who are at high risk for infection: the estimated 100,000 men and women who work on those farms. There has been no widespread testing to see how many may be infected. None have been vaccinated against bird flu.

That leaves the workers and their families vulnerable to a poorly tracked pathogen. And it poses broader public health risks. If the virus were to find its way into the wider population, experts say, dairy workers would be a likely route.

“We have no idea if this virus is going to evolve to become a pandemic strain, but we know today that farmworkers are being exposed, and we have good reasons to think that they are getting sick,” said Jennifer Nuzzo, director of the Pandemic Center at Brown University School of Public Health.

A majority of dairy farmworkers are Spanish-speaking immigrants, often undocumented, who may not have paid sick leave or be protected by occupational safety laws. They may lack access to medical providers, and their employers can be intolerant of absences.

“This sector of workers is not only at the very, very highest risk because they’re having that direct, intimate contact with discharge, raw milk, with infected animals, but they’re also at the very, very highest level of risk in terms of having no social safety net,” said Elizabeth Strater, an organizer with United Farm Workers.

Interviews with more than three dozen federal and state officials, public health experts, farmers and workers’ organizations show how little is known about what’s occurring on farms: how many workers may be affected, how the virus is evolving and how it is spreading among cows.

So far, the virus, called H5N1, has been detected in cattle herds in nine states. While veterinarians have said there are unconfirmed reports of farmworkers with flulike symptoms, only 30 have been tested as of Wednesday.

Barring extraordinary circumstances, state and federal health officials do not have the authority to demand access to farms. Instead, the Food and Drug Administration and the Department of Agriculture are testing milk and ground beef on grocery shelves for the virus.

The Centers for Disease Control and Prevention is waiting for infected people to show up at clinics.

“Do you want to find out about a virus when it’s spreading among people so much that they are coming into emergency rooms in hordes, or do you want to catch it on the farms so you can treat the people and slow the spread?” said Rick Bright, the chief executive of Bright Global Health, which focuses on responses to public health emergencies.

A tangled regulatory system complicates the situation, said Dr. Jay Varma, who served in the C.D.C.’s food-borne diseases branch and oversaw food safety as a deputy commissioner at New York City’s health department.

The agriculture department regulates large commercial farms and can mandate testing of animals — although it has not yet done so — but not of farmworkers. The department “doesn’t ever want to be in a position where it has to declare that food supply from the U.S. is unsafe, because some of those food products may be exported to other countries and that can have a huge economic impact,” Dr. Varma said.

The C.D.C. has authority over ports of entry into the United States, but domestically the agency needs state approval to do much of its work. The F.D.A., Occupational Safety and Health Administration, the Environmental Protection Agency and Citizenship and Immigration Services all have roles to play, but each has its bureaucratic layers and institutional culture.

This patchwork can be an impediment during a disease outbreak, some experts said. In 2009, the response to a cluster of bacterial infections in a salami product was delayed because the Department of Agriculture regulated the meat, the F.D.A. was responsible for the cracked black pepper that coated it and the C.D.C. was in charge of investigating the people who became ill.

Dr. Nirav Shah, principal deputy director of the C.D.C., dismissed the notion that bureaucracy was an obstacle as “overly simplistic,” and said the agencies responding to the outbreak talk numerous times a day to coordinate their activities and to work with state partners.

“This stuff is hard,” he said. But “we’re working together on this because we have common goals.”

Because of the relatively small number of cases — 36 affected herds out of some 26,000 nationwide and one infected farmworker — some farmers see the bird flu as a distant threat. Even those who support public health efforts are hesitant to let federal officials on their properties.

Jason Schmidt owns Grazing Plains Farm in Whitewater, Kan., “an itty bitty little” farm as he refers to it, with 70 dairy cows he raises himself. Mr. Schmidt said he supported the government’s public health role, but would not want officials traipsing around his farm.

If he were to see a sick cow, “I sure hope I’d report it,” he said. “But there’s that little devil on my shoulder saying, ‘Just keep quiet and divert that milk from those sick cows and put that down the drain and don’t say anything.’”

Mitch Breunig, who owns Mystic Valley Dairy in Sauk City, Wis., said that if his veterinarian determined it was “prudent,” he would test a cow with bird flu symptoms, but “I really don’t want the C.D.C. coming to my farm.”

So far, the outbreak has affected not small farms, but the giant dairies that increasingly dominate the industry and often rely on migrant workers.

The owners of such farms “don’t care about our health, they just care that we do our jobs,” said Luis Jimenez, who works on a dairy in upstate New York and founded a group supporting undocumented immigrant farmworkers.

“Their cows’ health is more important to them than their workers,” he added.

Farms are often geographically remote, and the workers — who sometimes live on site — may not have transportation to reach medical care. And to many, the kinds of symptoms ascribed to bird flu infection may not seem particularly urgent.

“We’re talking about an eye infection or a cough, and these are people who have back aches and arm aches and broken this and that,” said Monica Schoch-Spana, a medical anthropologist at the Johns Hopkins Center for Health Security.

Even without access to farms, health officials could support migrant workers in other settings, advocates noted. “They go to church, they go to stores, they go to restaurants, and there are other ways to reach them,” said Amy Liebman, a program officer at the Migrant Clinicians Network.

To make testing more attractive, community centers could offer other health care services, legal assistance and food, and educate workers on ways they can protect themselves and their families, Ms. Liebman said.

Dr. Shah said the C.D.C. was working with veterinarians and organizations like the Migrant Clinicians Network to reach farmworkers. “We, too, would like to offer testing to more workers,” he said.

On Monday, Dr. Shah asked that state health officials provide goggles, face shields and gloves to farmworkers, and collaborate with trusted community organizations to educate them on the importance of the gear in preventing infection.

Despite the risks to their health, farmworkers are not required to wear protective equipment. “It’s not a mandate, no one is being forced to do anything here,” Dr. Shah said.

But the nature of farm work and the settings in which it is done — milk parlors that quickly render masks wet and useless, for example — can make wearing protective equipment challenging.

A few states have taken steps to contain the outbreak, with limited success.

Texas offered to provide protective gear to dairies, but only four came forward, according to a spokesman for the state’s health department. Idaho has also offered protective equipment since the outbreak’s onset, but no farms have taken up the offer.

Idaho health officials have not asked to go onto farms “for privacy and biosecurity reasons,” Dr. Christine Hahn, the state epidemiologist, said in an email, although they helped to test one farmworker for the infection.

Michigan is prohibiting exhibition of dairy cows and poultry until the outbreak has subsided. The state does not require testing of cows or farmworkers.

The current situation has shown that dairy farms may seed new outbreaks that quickly spread, as has long been the case on poultry and pig farms, several experts said.

“If you had to hide a novel virus emergence in the United States, one of the best places to hide it would be in animal workers in rural America,” said Dr. Gregory Gray, an infectious disease epidemiologist at the University of Texas Medical Branch.

Surveillance of those workers is “not nearly as strong as we might see for other population groups,” he said.

To build surveillance networks that include farmworkers and their families, federal, state and local agencies will have to first establish trust, said Dr. Andrew Bowman, a veterinary epidemiologist at Ohio State University.

“If you look at the influenza surveillance we’ve done in swine, that didn’t happen overnight,” Dr. Bowman said. “That took a decade to build.”

While surveillance is important, some experts cautioned against testing farmworkers without first catering to their needs.

“If we prepare to collect information that’s only going to benefit others and not necessarily directly protect them, I just think that’s a very hard thing to do ethically,” Dr. Nuzzo said.

Miguel Salazar contributed reporting.